Fix \dRp+ output when describing publications with a lower server version.

The psql was not careful that the new column "Generated columns" won't be
present in the lower version. This was introduced in recent commit
7054186c4e.

Author: Vignesh C
Reviewed-by: Peter Smith
Discussion: https://postgr.es/m/CALDaNm3OcXdY0EzDEKAfaK9gq2B67Mfsgxu93+_249ohyts=0g@mail.gmail.com

src/bin/psql/describe.c

@@ -6495,12 +6495,23 @@ describePublications(const char *pattern)
 	if (has_pubtruncate)
 		appendPQExpBufferStr(&buf,
 							 ", pubtruncate");
+	else
+		appendPQExpBufferStr(&buf,
+							 ", false AS pubtruncate");
+
 	if (has_pubgencols)
 		appendPQExpBufferStr(&buf,
 							 ", pubgencols");
+	else
+		appendPQExpBufferStr(&buf,
+							 ", false AS pubgencols");
+
 	if (has_pubviaroot)
 		appendPQExpBufferStr(&buf,
 							 ", pubviaroot");
+	else
+		appendPQExpBufferStr(&buf,
+							 ", false AS pubviaroot");
 
 	appendPQExpBufferStr(&buf,
 						 "\nFROM pg_catalog.pg_publication\n");