Avoid crash in selectCurrentWord

When viewing the context menu and choosing "Select word" on a
WebTextView with default value which is reset by a java script, there
will be a null pointer exception. One example of this is:

http://m.sonyericsson.com/mobile/online/home?lang=en-gb&os=android

Since no characters are yet entered, it does not make sense to offer
the option to select a word. This fix changes the context menu so that
"Select word" appears only if there is a
SelectionModifierCursorController. (the text is no longer empty).

Change-Id: I94b35e39b0ff9641bbf390b9a2510be437d3e6ed