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/ClientGameRunner.ts

@@ -558,12 +558,11 @@ export class ClientGameRunner {
       this.myPlayer = myPlayer;
     }
     this.myPlayer.actions(tile).then((actions) => {
-      if (this.myPlayer === null) return;
       if (actions.canAttack) {
         this.eventBus.emit(
           new SendAttackIntentEvent(
             this.gameView.owner(tile).id(),
-            this.myPlayer.troops() * this.renderer.uiState.attackRatio,
+            this.myPlayer!.troops() * this.renderer.uiState.attackRatio,
           ),
         );
       } else if (this.canAutoBoat(actions, tile)) {
@@ -674,12 +673,11 @@ export class ClientGameRunner {
     }
 
     this.myPlayer.actions(tile).then((actions) => {
-      if (this.myPlayer === null) return;
       if (actions.canAttack) {
         this.eventBus.emit(
           new SendAttackIntentEvent(
             this.gameView.owner(tile).id(),
-            this.myPlayer.troops() * this.renderer.uiState.attackRatio,
+            this.myPlayer!.troops() * this.renderer.uiState.attackRatio,
           ),
         );
       }