QA: minor code tweak (#282)

This fixes the following error from PHPStan:
```
 ------ ---------------------------------------------------------
  Line   Lib/Helpers.php
 ------ ---------------------------------------------------------
  758    Cannot access offset int on 0|0.0|''|'0'|array{}|false.
 ------ ---------------------------------------------------------
```

Co-authored-by: jrfnl <jrfnl@users.noreply.github.com>

VariableAnalysis/Lib/Helpers.php

@@ -755,7 +755,7 @@ public static function getListAssignments(File $phpcsFile, $listOpenerIndex)
 				$parentSquareBracket = self::findContainingOpeningSquareBracket($phpcsFile, $listOpenerIndex);
 				if (is_int($parentSquareBracket)) {
 					// Collect the opening index, but we don't actually need the closing paren index so just make that 0
-					$parents[$parentSquareBracket] = 0;
+					$parents = [$parentSquareBracket => 0];
 				}
 			}
 			// If we have no parents, this is not a nested assignment and therefore is not an assignment