Fix loading map names with a numeric name

[DanVanAtta]

Fixes loading maps with names like '1941'.

When stored in game data properties, the value at some point
turns into an Integer. When extracting we attempt to cast this
to a 'String' and this then causes a ClassCastException. To fix
this, we can instead do a 'String.valueOf' rather than do an
explicit cast.

Testing

Screens Shots

Additional Notes to Reviewer

Release Note

FIX|Games with numeric names, like '1941', would not load.

[codecov]

Codecov Report

Merging #8774 (4bafdf7) into master (caa2068) will not change coverage.
The diff coverage is 0.00%.

@@            Coverage Diff            @@
##             master    #8774   +/-   ##
=========================================
  Coverage     25.24%   25.24%           
- Complexity     7502     7503    +1     
=========================================
  Files          1297     1297           
  Lines         80768    80768           
  Branches      11048    11048           
=========================================
  Hits          20391    20391           
+ Misses        58289    58288    -1     
- Partials       2088     2089    +1     

Impacted Files	Coverage Δ	Complexity Δ
...main/java/games/strategy/triplea/ui/UiContext.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...a/ui/unit/scroller/AvatarCoordinateCalculator.java	69.23% <0.00%> (-15.39%)	5.00% <0.00%> (-1.00%)
...egate/power/calculator/MainDefenseCombatValue.java	76.08% <0.00%> (+2.17%)	11.00% <0.00%> (+2.00%)

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