Relax changes

[alexander-yevsyukov]

This PR relaxes validation requirements for StringChange and BytesChange types. Previously, new_value fields were required. It was so to encourage introducing commands that would start from Clear (e.g. ClearDescription) instead of having a command that would just accept a blank new value.

The intention was good, but it introduced the following problems:

1. It's too much headache when it comes to editing values that we don't want to die for. E.g. a text field may be cleared just because the previous one was outdate and the user did not invent a new one non-blank one. Requiring having special handling for empty values with a separate command and event is too much.
2. We cannot require new_value for fields of simple types. Such inconsistency makes the problem no.1 is even bigger.

[codecov]

Codecov Report

Merging #1307 into master will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #1307      +/-   ##
============================================
- Coverage     91.09%   91.06%   -0.03%     
+ Complexity     4746     4744       -2     
============================================
  Files           608      608              
  Lines         15089    15088       -1     
  Branches        854      854              
============================================
- Hits          13745    13740       -5     
- Misses         1078     1080       +2     
- Partials        266      268       +2