Silence -Wmaybe-uninitialized compiler warning in dbcommands.c.

Introduced in f2553d4306. See also 3f6b3be39c, which did so for nearby
variables.

Discussion: https://postgr.es/m/20220319014707.kgtomqdzm6m2ulro@alap3.anarazel.de

src/backend/commands/dbcommands.c

@@ -109,7 +109,7 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
 	char	   *src_collate = NULL;
 	char	   *src_ctype = NULL;
 	char	   *src_iculocale = NULL;
-	char		src_locprovider;
+	char		src_locprovider = '\0';
 	char	   *src_collversion = NULL;
 	bool		src_istemplate;
 	bool		src_allowconn;