* prevent firefox marking required textareas invalid
Bug was caused by an IE10/IE11 bugfix dealing with the placeholder attribute and textContent. Solved by avoiding the IE bugfix when textContent was empty.
Closes#16402
* more explicit conditional check for textContent
re: @philipp-spiess code review
* clarify textarea test fixture's expected result
better describe the behavior we are testing for
re: @philipp-spiess code review