Warn if <If> or <Else> or <For> are empty

[texttechne]

In the next release an empty control statement, i.e. without child element, will not throw an error. However, a warning makes sense in these cases.

[jimfb]

I'm not actually sure if it makes sense to warn in such cases. It would be like if function(){} warned in javascript. Obviously that is a suspicious expression, but it's a perfectly valid/legitimate one.

I argue that it's really easy to see that the <if></if> block is empty, so the warning isn't helping the user catch a subtle mistake. And it's also not detrimental to the application (ie. a memory leak is detrimental), so I'm not sure there is any benefit to emitting a warning in this situation.

[texttechne]

Convinced.

[vkbansal]

The warning level can be controlled by setting jsx-if-single-child value to 1 (for warning) or 2(for error).
As for allowing empty <If></If>, blocks, I'm open to adding a setting/option.