Incorrect validation of custom multiplier preprocessing step parameter

Steps to reproduce:

1. Create an item and add preprocessing step of type "Custom multiplier".
2. Add some number with space after it (like "10     ").
3. Try to save item. Observe error telling that numeric value is expected in parameter field.
4. Now, move empty space from end of multiplier to the beginning (like "   10").
5. Save changes.

Result:
Item successfully saved.