There should be no image previews when the URL is empty in image dialog

[martinkou]

To reproduce:

1. Open replacebyclass sample in IE.
2. Open the image dialog.
3. Even before any URL is filled into the dialog box, we're still having a cross symbol in the preview pane.

Or, alternatively

1. Open replacebyclass sample in any browser.
2. Open the image dialog.
3. Fill in any URL and defocus the URL text box.
4. Remove the URL
5. The image is still in the preview pane - it should be cleared instead.

[garry.yao]

It doesn't work for IE.

[tobiasz.cudnik]

Seem that setAttribute( 'name', null ) doesn't work like removeAttribute( 'name' ) in IE, while doing so in other browsers.

[garry.yao]

How about using the below codes after #3898's fix?

if( this.getContentElement( 'info', 'txtUrl' ).validate() != true )
// instead of
if ( !CKEDITOR.tools.trim( this.getValueOf( 'info', 'txtUrl' ) ) )

[tobiasz.cudnik]

Using validate() doesn't work for first dialog show, while getValueOf() is not affected. I will check why that is happening.

[tobiasz.cudnik]

After updating dialog file to newer revision it indeed works. Attaching new patch.

[garry.yao]

R+ for 3897_2, 3897_3 instead will bring a validation alert which is not acceptable.

[tobiasz.cudnik]

Fixed with [4179].

In future will can use 3897_3.patch, when dialog validation system will allow for it.