The find/replace dialog and overlay do not allow to change the "whole
word" search option when the input field is empty. There is, however, no
reason for disabling the option when the input field is empty. With this
change, the "whole word" option can also be activated and deactivated
when the find input is empty. In addition, the enablement behavior of
the option in the find/replace overlay is unified.

Fixes https://github.com/eclipse-platform/eclipse.platform.ui/issues/2162