minor #18400 Lower complexity of Form:isValid() (krzysiekpiasecki)

This PR was squashed before being merged into the 2.7 branch (closes #18400).

Discussion
----------

Lower complexity of Form:isValid()

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Replace #18391
- use Yoda condition,
- merge into 2.7 branch

Why?

- lower LOC
- less return expressions
- lower complexity
- more readable comparison

Commits
-------

d583ec3 Lower complexity of Form:isValid()

src/Symfony/Component/Form/Form.php

@@ -766,11 +766,7 @@ public function isValid()
             return true;
         }
 
-        if (count($this->getErrors(true)) > 0) {
-            return false;
-        }
-
-        return true;
+        return 0 === count($this->getErrors(true));
     }
 
     /**