Sanitize abstractTextField propagated DOM value

In general, we don't want to present undefined, nulls, etc. as anything
more than empty strings.
Sanitize abstractTextArea propagated DOM value

Same behavior of the textField now, though they should probably share
a common base. It's suspicious that they don't at least share
abstractControl.
Merge pull request #1227 from mczepiel/text-field

Sanitize abstractTextField propagated DOM value