Fix problem with noSelection-option by improving find of emptyValue

store.find didn't found the emptyValue and added a lot more entries
than one. Now the function is searching for .emptyValue=true
Merge pull request #1 from Ben-Ho/master-no-selection

Fix problem with noSelection-option by improving find of emptyValue
Also check for undefined

Else if value is undefined a filter without value is added which causes
and exception
Merge pull request #2 from Ben-Ho/master-combobox-no-value

Also check for undefined