Generic/DisallowYodaConditions: add comment documenting uncovered line

This commits adds a code comment to documment that the line below it cannot
be covered by tests and cannot be removed as external sniffs may call
`DisallowYodaConditionsSniff::isArrayStatic()` directly.

Author: rodrigoprimo

src/Standards/Generic/Sniffs/ControlStructures/DisallowYodaConditionsSniff.php

@@ -146,6 +146,7 @@ public function isArrayStatic(File $phpcsFile, $arrayToken)
             $start = $tokens[$arrayToken]['parenthesis_opener'];
             $end   = $tokens[$arrayToken]['parenthesis_closer'];
         } else {
+            // Shouldn't be possible but may happen if external sniffs are using this method.
             return true;
         }