FilteringSelect: placeHolder was disappeared after set Null

[vtsuper]

when I programmatically set value of a dijit.form.FilteringSelect(with placeHolder) by this

w.set('value',); or w.set('value',null);

the value will change but the placeHolder will not show again.

to reproduce this case

1. load the sample file and you will see the dijit.form.FilteringSelect with placeHolder
2. select any options
3. select set Null, blank or undefined
4. although the option was change to blank but the placeHolder won't show again.

[Douglas Hays]

Already fixed in 1.7.

[Douglas Hays]

In [27372]:

Refs #14229. Add automated placeHolder test to prevent any future regression.