Enable placeholders when model is cleared

[haohong]

Fixed the following things:

• Setting model to null doesn't trigger all components to be cleared when part of date components are set. For example, the ngModel is already set null, and user selects year and month, not date. In that case, there is no means to reset all date components(year, month, date) to blank state.
• Setting ngModel to null should enable placeholders back.

[jfmdev]

Thanks for your contribution!