fix(ui5-button): determine icon-only ignoring comment nodes

[ilhan007]

If we use the "ui5-button" internally in other component's template like this:

<ui5-button icon="{{this.icon}}">{{this.text}}</ui5-button>

And, if any button happens not to have a text, we will get the following result in the DOM:

<ui5-button icon="home"><!----><!----></ui5-button>

Then, in Button.js we used to set icon-only if there are no childNodes, but in this case there are child nodes, because the comments are child nodes. Some styles, that rely on having a text are applied and the button does not look right:

Now the icon-only is set properly, checking against the textContent as it does not include comments and the same button looks like a standard one:

Other possible way is to filter the child nodes by nodeType:

this.iconOnly = !Array.from(this.childNodes)
                       .filter(node => node.nodeType !== Node.COMMENT_NODE).length;