fix(ui): Fix PHP warning for non-variables passed by reference (#1080)

* Fix notice about variables passed by reference

`reset` only takes variables so we can not pass the result of
`array_keys` directly but have to use an intermediate variable.

* Merge variable fallback expressions

`array_key_exists('schema', $input) ? $input['schema'] : NULL` is equal
to `$input['schema'] ?? NULL` on PHP versions that support the
null-coalescing operator. Since we already use this elsewhere we already
require such PHP versions.

This allows us to substitute the simplified one-use variable leading to
`$input['schema'] ?? NULL ?? $server->get('schema')`. The expression `??
NULL ??` can be safely transformed to `??` to result in the simplified
expression in this commit.

Co-authored-by: Klaus Purer <[email protected]>

Author: Kingdutch

src/Form/ServerForm.php

@@ -90,10 +90,12 @@ public function form(array $form, FormStateInterface $formState) {
     $schemas = array_map(function ($definition) {
       return $definition['name'] ?? $definition['id'];
     }, $this->schemaManager->getDefinitions());
+    $schema_keys = array_keys($schemas);
 
     $input = $formState->getUserInput();
-    $inputSchema = array_key_exists('schema', $input) ? $input['schema'] : NULL;
-    $schema = ($inputSchema ?? $server->get('schema')) ?: reset(array_keys($schemas));
+    // Use the schema selected by the user, the one configured, or fall back to
+    // the first schema that is defined.
+    $schema = ($input['schema'] ?? $server->get('schema')) ?: reset($schema_keys);
 
     if ($this->operation == 'add') {
       $form['#title'] = $this->t('Add server');