JS: return an empty string instead of null from 'getToolTipText' for EditBox
JS: return an empty string instead of null from 'getToolTipText' for EditBox

Fixes a bug that occasionally caused a null pointer exception in
ToolTipManager. It should be okay to return null, but returning an empty
string seems to work more reliably in any case.

Signed-off-by: Harry King <409587b9e6671aa0763646191d292852dc49a658@gmail.com>

jaamsim/jaamsim,jaamsim/jaamsim,jaamsim/jaamsim,jaamsim/jaamsim