Simplified DateTimeImmutable::format(): always returns a string

Also:

 * a non-empty format string will always lead to `non-empty-string`
 * it seems that you can throw **everything** at `DateTimeInterface#format()`, even null bytes,
   and it will always produce a `string`

stubs/CoreImmutableClasses.phpstub

@@ -15,7 +15,7 @@ class DateTimeImmutable implements DateTimeInterface
      *
      * @param string $format
      *
-     * @return (\PHP_MAJOR_VERSION is int<0, 7> ? string|false : string)
+     * @return ($format is non-empty-string ? non-empty-string : string)
      */
     public function format($format) {}
 

tests/MethodCallTest.php

@@ -271,8 +271,8 @@ class MethodCallTest extends TestCase
 
                     $b = (new DateTimeImmutable())->modify("+3 hours");',
                 'assertions' => [
-                    '$yesterday' => 'MyDate',
-                    '$b' => 'DateTimeImmutable',
+                    '$yesterday' => 'MyDate|false',
+                    '$b' => 'DateTimeImmutable|false',
                 ],
             ],
             'magicCall' => [