Cleanup/refactor: Remove some redundant checks (#3235)

## Description:

PR 3/x in effort to break up PR
https://github.com/openfrontio/OpenFrontIO/pull/3220. Follows on already
merged https://github.com/openfrontio/OpenFrontIO/pull/3233 and
https://github.com/openfrontio/OpenFrontIO/pull/3234.

Please see if these can be merged for v30.

- **ClientGameRunner**: removed two redundant myPlayer===null checks
since that was already done right above, instead use !.
- **BuildMenu**: just like in UnitDisplay, assign public PlayerActions
default value of null. So that in canCreateOrBuild, where we already do
a === null check on it btw, we can safely skip the assignment to const
buildableUnits and just directly loop over
this.playerActions.buildableUnits.
- **RadialMenuElements**: don't call canBuildOrUpgrade 3x in
CreateMenuElements for the .map on flattenedBuildTable, instead do it
once and re-use outcome.

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

## Please put your Discord username so you can be contacted if a bug or
regression is found:

tryout33

src/client/graphics/layers/BuildMenu.ts

@@ -127,7 +127,7 @@ export class BuildMenu extends LitElement implements Layer {
   public eventBus: EventBus;
   public uiState: UIState;
   private clickedTile: TileRef;
-  public playerActions: PlayerActions | null;
+  public playerActions: PlayerActions | null = null;
   private filteredBuildTable: BuildItemDisplay[][] = buildTable;
   public transformHandler: TransformHandler;
 
@@ -361,8 +361,9 @@ export class BuildMenu extends LitElement implements Layer {
     if (this.game?.myPlayer() === null || this.playerActions === null) {
       return false;
     }
-    const buildableUnits = this.playerActions?.buildableUnits ?? [];
-    const unit = buildableUnits.filter((u) => u.type === item.unitType);
+    const unit = this.playerActions.buildableUnits.filter(
+      (u) => u.type === item.unitType,
+    );
     if (unit.length === 0) {
       return false;
     }