Consistently use NULL for invalid GUC unit strings

Patch by Euler Taveira

src/backend/utils/misc/guc.c

@@ -7734,7 +7734,7 @@ GetConfigOptionByNum(int varnum, const char **values, bool *noshow)
 				values[2] = "min";
 				break;
 			default:
-				values[2] = "";
+				values[2] = NULL;
 				break;
 		}
 	}