If listbox with an empty content, the head's size in Opera is wrong

Related to this test case - zktest/test2/B30-1823236.zul. (or B30-1823236.ztl)

If the content is empty, in MeshWidget.js we cannot use the this._getEbodyWd() to get the width for Opera.

For example,
var tblwd = this._getEbodyWd() // <- which use this.ebody.clientWidth;

In Opera, if the content is empty, it will return a size which contains a scrollbar width, but it is unnecessary.

To fix this we can check if the this.ebody.offsetHeight is 0, then return this.ebody.offsetWidth for Opera to avoid this issue.