psql: fix \l display for pre-v15 databases.

With a pre-v15 server, show NULL for the "ICU Locale" column,
matching what you see in v15 when the database locale isn't ICU.
The previous coding incorrectly repeated datcollate here.

(There's an unfinished discussion about whether to consolidate
these columns in \l output, but in any case we'd want this fix
for \l+ output.)

Euler Taveira, per report from Christoph Berg

Discussion: https://postgr.es/m/YlmIFCqu+TZSW4rB@msg.df7cb.de

src/bin/psql/describe.c

@@ -904,7 +904,7 @@ listAllDbs(const char *pattern, bool verbose)
 						  gettext_noop("Locale Provider"));
 	else
 		appendPQExpBuffer(&buf,
-						  "       d.datcollate as \"%s\",\n"
+						  "       NULL as \"%s\",\n"
 						  "       'libc' AS \"%s\",\n",
 						  gettext_noop("ICU Locale"),
 						  gettext_noop("Locale Provider"));