[sql mode] Set lineComment to "--" when no other style is used

mode/sql/sql.js

@@ -193,7 +193,7 @@ CodeMirror.defineMode("sql", function(config, parserConfig) {
 
     blockCommentStart: "/*",
     blockCommentEnd: "*/",
-    lineComment: support.commentSlashSlash ? "//" : support.commentHash ? "#" : null
+    lineComment: support.commentSlashSlash ? "//" : support.commentHash ? "#" : "--"
   };
 });