libpq: have PQconnectdbParams() and PQpingParams accept "" as default

Previously, these functions treated "" optin values as defaults in some
ways, but not in others, like when comparing to .pgpass.  Also, add
documentation to clarify that now "" and NULL use defaults, like
PQsetdbLogin() has always done.

BACKWARD INCOMPATIBILITY

Patch by Adrian Vondendriesch, docs by me

Report by Jeff Janes

doc/src/sgml/libpq.sgml

@@ -131,15 +131,15 @@ PGconn *PQconnectdbParams(const char * const *keywords,
       <para>
        The passed arrays can be empty to use all default parameters, or can
        contain one or more parameter settings. They should be matched in length.
-       Processing will stop with the last non-<symbol>NULL</symbol> element
-       of the <literal>keywords</literal> array.
+       Processing will stop at the first <symbol>NULL</symbol> element
+       in the <literal>keywords</literal> array.
       </para>
 
       <para>
-       If  any  parameter is unspecified, then the corresponding
-       environment variable (see <xref linkend="libpq-envars">)
-       is checked. If the  environment  variable is not set either,
-       then the indicated built-in defaults are used.
+       If  any  parameter is NULL or an emptry string, the corresponding
+       environment variable (see <xref linkend="libpq-envars">) is checked.
+       If the  environment  variable is not set either, then the indicated
+       built-in defaults are used.
       </para>
 
       <para>

src/interfaces/libpq/fe-connect.c

@@ -4357,7 +4357,7 @@ conninfo_array_parse(const char *const * keywords, const char *const * values,
 		const char *pname = keywords[i];
 		const char *pvalue = values[i];
 
-		if (pvalue != NULL)
+		if (pvalue != NULL && pvalue[0] != '\0')
 		{
 			/* Search for the param record */
 			for (option = options; option->keyword != NULL; option++)