Bluetooth : UI bug fix : Pairing dialog box takes space as character

- In pairing dialog box even if PIN was removed in text box, still the OK button is enabled
- When length is zero , disable the OK button in pairing dialog
- update fix to prevent error while mOkButton is not ceated.

Change-Id: I38ff3029cff2e7c4e87b7f9c93fa262125e91d44
Signed-off-by: jhtop.kim <jhtop.kim@samsung.com>