fixed bug #2399
added check expression for empty string
Merge branch 'master' into haoxuan2
fixed sticky button issue
fixed check equation box with unknown variable in student mode issue
Code review for "sticky" clicks and equations with
unknown variables.  Combined "toPM" and "cancelUpdate" to
mean that the student entry is neither sent to the PM or
saved in the model.
Merge branch 'haoxuan2'