Allow empty column labels

[gratex]

No description provided.

[kfranqueiro]

This was a good idea, but rather than require people to specify blank labels as null, which feels unnatural, I've fixed the code in Grid, GridFromHtml, and selector to allow specifying an actual empty string for a blank label.

[gratex]

My solution was also accepting empty strings for empty labels (and NULL for using column.field as fallback - same as in previous implementation).
Now if you specify NULL as label, you will have "null" string in the header.

But thank you anyway, usage with empty string is working.