User has to enter at least one character to be able to save the sensor's new name

Build tested: 198ed59

**Steps**
Tap on a sensor to favorite it
Tap directly on Save button

**Expected**
The Save/Done button is disabled till user enter at least one char, then the button is active and the new name of the sensor is shown. 

**Actual**
The Save/Done button is always active. User can tap on it without entering any letter. The sensor will be named as 'Sensor'
