Make construct_[md_]array return a valid empty array for zero-size input.

If construct_array() or construct_md_array() were given a dimension of
zero, they'd produce an array that contains no elements but has positive
dimension.  This violates a general expectation that empty arrays should
have ndims = 0; in particular, while arrays like this print as empty,
they don't compare equal to other empty arrays.

Up to now we've expected callers to avoid making such calls and instead
be careful to call construct_empty_array() if there would be no elements.
But this has always been an easily missed case, and we've repeatedly had to
fix callers to do it right.  In bug #14826, Erwin Brandstetter pointed out
yet another such oversight, in ts_lexize(); and a bit of examination of
other call sites found at least two more with similar issues.  So let's
fix the problem centrally and permanently by changing these two functions
to construct a proper zero-D empty array whenever the array would be empty.

This renders a few explicit calls of construct_empty_array() redundant,
but the only such place I found that really seemed worth changing was in
ExecEvalArrayExpr().

Although this fixes some very old bugs, no back-patch: the problem is
pretty minor and the risk of changing behavior seems to outweigh the
benefit in stable branches.

Discussion: https://postgr.es/m/20170923125723.1448.39412@wrigleys.postgresql.org
Discussion: https://postgr.es/m/20570.1506198383@sss.pgh.pa.us

src/backend/executor/execExprInterp.c

@@ -2131,14 +2131,6 @@ ExecEvalArrayExpr(ExprState *state, ExprEvalStep *op)
 		Datum	   *dvalues = op->d.arrayexpr.elemvalues;
 		bool	   *dnulls = op->d.arrayexpr.elemnulls;
 
-		/* Shouldn't happen here, but if length is 0, return empty array */
-		if (nelems == 0)
-		{
-			*op->resvalue =
-				PointerGetDatum(construct_empty_array(element_type));
-			return;
-		}
-
 		/* setup for 1-D array of the given length */
 		ndims = 1;
 		dims[0] = nelems;

src/backend/utils/adt/arrayfuncs.c

@@ -3297,6 +3297,7 @@ array_map(FunctionCallInfo fcinfo, Oid retType, ArrayMapState *amstate)
  *
  * A palloc'd 1-D array object is constructed and returned.  Note that
  * elem values will be copied into the object even if pass-by-ref type.
+ * Also note the result will be 0-D not 1-D if nelems = 0.
  *
  * NOTE: it would be cleaner to look up the elmlen/elmbval/elmalign info
  * from the system catalogs, given the elmtype.  However, the caller is
@@ -3331,6 +3332,7 @@ construct_array(Datum *elems, int nelems,
  *
  * A palloc'd ndims-D array object is constructed and returned.  Note that
  * elem values will be copied into the object even if pass-by-ref type.
+ * Also note the result will be 0-D not ndims-D if any dims[i] = 0.
  *
  * NOTE: it would be cleaner to look up the elmlen/elmbval/elmalign info
  * from the system catalogs, given the elmtype.  However, the caller is
@@ -3362,12 +3364,12 @@ construct_md_array(Datum *elems,
 				 errmsg("number of array dimensions (%d) exceeds the maximum allowed (%d)",
 						ndims, MAXDIM)));
 
-	/* fast track for empty array */
-	if (ndims == 0)
-		return construct_empty_array(elmtype);
-
 	nelems = ArrayGetNItems(ndims, dims);
 
+	/* if ndims <= 0 or any dims[i] == 0, return empty array */
+	if (nelems <= 0)
+		return construct_empty_array(elmtype);
+
 	/* compute required space */
 	nbytes = 0;
 	hasnulls = false;

src/test/regress/expected/tsearch.out

@@ -618,6 +618,17 @@ SELECT * from ts_debug('english', '5aew.werc.ewr:8100/?xx');
  url_path | URL path    | /?xx                   | {simple}     | simple     | {/?xx}
 (3 rows)
 
+SELECT token, alias,
+  dictionaries, dictionaries is null as dnull, array_dims(dictionaries) as ddims,
+  lexemes, lexemes is null as lnull, array_dims(lexemes) as ldims
+from ts_debug('english', 'a title');
+ token |   alias   |  dictionaries  | dnull | ddims | lexemes | lnull | ldims 
+-------+-----------+----------------+-------+-------+---------+-------+-------
+ a     | asciiword | {english_stem} | f     | [1:1] | {}      | f     | 
+       | blank     | {}             | f     |       |         | t     | 
+ title | asciiword | {english_stem} | f     | [1:1] | {titl}  | f     | [1:1]
+(3 rows)
+
 -- to_tsquery
 SELECT to_tsquery('english', 'qwe & sKies ');
   to_tsquery   

src/test/regress/sql/tsearch.sql

@@ -145,6 +145,10 @@ SELECT * from ts_debug('english', 'http://www.harewoodsolutions.co.uk/press.aspx
 SELECT * from ts_debug('english', 'http://aew.wer0c.ewr/id?ad=qwe&dw<span>');
 SELECT * from ts_debug('english', 'http://5aew.werc.ewr:8100/?');
 SELECT * from ts_debug('english', '5aew.werc.ewr:8100/?xx');
+SELECT token, alias,
+  dictionaries, dictionaries is null as dnull, array_dims(dictionaries) as ddims,
+  lexemes, lexemes is null as lnull, array_dims(lexemes) as ldims
+from ts_debug('english', 'a title');
 
 -- to_tsquery