bug #30028 [Form] fix some docblocks and type checks (xabbuh)

This PR was merged into the 3.4 branch.

Discussion
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[Form] fix some docblocks and type checks

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #30018
| License       | MIT
| Doc PR        |

Commits
-------

ec0c37192b [Form] fix some docblocks and type checks

Author: fabpot

AbstractType.php

@@ -52,7 +52,7 @@ public function configureOptions(OptionsResolver $resolver)
      */
     public function getBlockPrefix()
     {
-        return StringUtil::fqcnToBlockPrefix(\get_class($this));
+        return StringUtil::fqcnToBlockPrefix(\get_class($this)) ?: '';
     }
 
     /**