DevTools: do not throw error to console on AddNewRule

Obviously, an empty text is a valid text for a stylesheet. No need to
throw.

BUG=505358
R=pfeldman

Review URL: https://codereview.chromium.org/1212643005

git-svn-id: svn://svn.chromium.org/blink/trunk@198010 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/devtools/front_end/sdk/CSSStyleModel.js

@@ -1782,7 +1782,7 @@ WebInspector.CSSStyleSheetHeader.prototype = {
          */
         function textCallback(error, text)
         {
-            if (error || !text) {
+            if (error || text === null) {
                 WebInspector.console.error("Failed to get text for stylesheet " + this.id + ": " + error)
                 text = "";
                 // Fall through.