Get mean of actual values (do not imply missing == 0) in Streetscape

[martinfleis]

resolves #673

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.1%. Comparing base (4037c70) to head (7213ef5).
Report is 100 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #675     +/-   ##
=======================================
+ Coverage   97.4%   98.1%   +0.8%     
=======================================
  Files         26      40     +14     
  Lines       4328    7208   +2880     
=======================================
+ Hits        4214    7074   +2860     
- Misses       114     134     +20     

Files with missing lines	Coverage Δ
momepy/streetscape.py	97.6% <100.0%> (ø)

... and 1 file with indirect coverage changes