Don't draw text only when width or height is 0

It can fix this issue:
Text is empty string '', but have fixedWidth and fixedHeight, with backgroundColor setting.

Author: rexrainbow

src/gameobjects/text/static/Text.js

@@ -185,7 +185,7 @@ var Text = new Class({
          * @private
          * @since 3.12.0
          */
-        this._text = '';
+        this._text = undefined;
 
         /**
          * Specify a padding value which is added to the line width and height when calculating the Text size.

src/gameobjects/text/static/TextCanvasRenderer.js

@@ -21,10 +21,12 @@
  */
 var TextCanvasRenderer = function (renderer, src, interpolationPercentage, camera, parentMatrix)
 {
-    if (src.text !== '')
+    if ((src.width === 0) || (src.height === 0))
     {
-        renderer.batchSprite(src, src.frame, camera, parentMatrix);
+        return;
     }
+
+    renderer.batchSprite(src, src.frame, camera, parentMatrix);
 };
 
 module.exports = TextCanvasRenderer;

src/gameobjects/text/static/TextWebGLRenderer.js

@@ -23,7 +23,7 @@ var Utils = require('../../../renderer/webgl/Utils');
  */
 var TextWebGLRenderer = function (renderer, src, interpolationPercentage, camera, parentMatrix)
 {
-    if (src.text === '')
+    if ((src.width === 0) || (src.height === 0))
     {
         return;
     }