Don't set empty placeholder to work around IE11 bug

[gaearon]

Should probably fix #11172.
Based on 15 branch code:

react/src/renderers/dom/shared/ReactDOMComponent.js

Lines 830 to 839 in 571a920

	// Avoid setting textContent when the text is empty. In IE11 setting
	// textContent on a text area will cause the placeholder to not
	// show within the textarea until it has been focused and blurred again.
	// https://github.com/facebook/react/issues/6731#issuecomment-254874553
	if (contentToUse !== '') {
	if (__DEV__) {
	setAndValidateContentChildDev.call(this, contentToUse);
	}
	DOMLazyTree.queueText(lazyTree, contentToUse);
	}

I made the fix more targeted to make it clearer.

I haven't tested because I don't have IE.

[aweary]

@gaearon I added a test case for textarea placeholders and deployed the DOM fixtures with this build here: http://placeholder-fix-ie11.surge.sh/textareas

It looks like the placeholder is now correctly rendered in IE11 on Windows 10.