[2048] Fix bug in legal action mask

[sotetsuk]

Legal action mask is wrong. In this case, agent cannot select action left (0) but currently it's legal action.

    [[ 8  2  0  0]
     [ 8  0  0  0]
     [ 8  0  0  0]
     [ 8  0  0  0]]

• Fix above bug
• legal action mask at init state

[codecov]

Codecov Report

Merging #1049 (51eab94) into main (2af728c) will decrease coverage by 0.47%.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1049      +/-   ##
==========================================
- Coverage   88.05%   87.58%   -0.47%     
==========================================
  Files          54       54              
  Lines        5908     5913       +5     
==========================================
- Hits         5202     5179      -23     
- Misses        706      734      +28     

Files	Coverage Δ
pgx/play2048.py	95.87% <100.00%> (+0.27%)	⬆️

... and 4 files with indirect coverage changes

[OkanoShinri]

確かに……
できた気になっていました🙇