fix(ui5-step-input): firing step input once when value is deleted

[tsanislavgatev]

fixes: #3457

[fifoosid]

This fix breaks the following case:
Enter 0 as value
Focus out the step input(event is thrown)
Delete the value
Focus out
Value is now empty string (should be 0)

[fifoosid]

This is a workaround for the previous comment and the following case is not handled:

Start with Stepinput with some value(for example: 5)
Mark the value & delete it
Focus out
The value 0 is restored(The value 5 should be restored).

[tsanislavgatev]

This is a workaround for the previous comment and the following case is not handled:

Start with Stepinput with some value(for example: 5)
Mark the value & delete it
Focus out
The value 0 is restored(The value 5 should be restored).

The current behaviour is when you have any value and you delete it( the state is empty string), then you focus out, you have 0 as value.