file_fdw: Improve error message

The hint looked a bit confusing when there were no valid options to
hint about.

contrib/file_fdw/file_fdw.c

@@ -230,8 +230,10 @@ file_fdw_validator(PG_FUNCTION_ARGS)
 			ereport(ERROR,
 					(errcode(ERRCODE_FDW_INVALID_OPTION_NAME),
 					 errmsg("invalid option \"%s\"", def->defname),
-					 errhint("Valid options in this context are: %s",
-							 buf.data)));
+					 buf.len > 0
+					 ? errhint("Valid options in this context are: %s",
+							   buf.data)
+					 : errhint("There are no valid options in this context.")));
 		}
 
 		/*

contrib/file_fdw/output/file_fdw.source

@@ -114,13 +114,13 @@ SELECT * FROM text_csv;
 -- force_not_null is not allowed to be specified at any foreign object level:
 ALTER FOREIGN DATA WRAPPER file_fdw OPTIONS (ADD force_not_null '*'); -- ERROR
 ERROR:  invalid option "force_not_null"
-HINT:  Valid options in this context are: 
+HINT:  There are no valid options in this context.
 ALTER SERVER file_server OPTIONS (ADD force_not_null '*'); -- ERROR
 ERROR:  invalid option "force_not_null"
-HINT:  Valid options in this context are: 
+HINT:  There are no valid options in this context.
 CREATE USER MAPPING FOR public SERVER file_server OPTIONS (force_not_null '*'); -- ERROR
 ERROR:  invalid option "force_not_null"
-HINT:  Valid options in this context are: 
+HINT:  There are no valid options in this context.
 CREATE FOREIGN TABLE tbl () SERVER file_server OPTIONS (force_not_null '*'); -- ERROR
 ERROR:  invalid option "force_not_null"
 HINT:  Valid options in this context are: filename, format, header, delimiter, quote, escape, null, encoding