Fix various minor issues found by Psalm

Author: PhrozenByte

src/Constraint/AssociativeArray.php

@@ -61,7 +61,7 @@ class AssociativeArray extends Constraint
     public function __construct(array $constraints, bool $allowMissing = false, bool $allowAdditional = true)
     {
         $isNoConstraint = static function ($constraint): bool { return !($constraint instanceof Constraint); };
-        if (!(is_array($constraints) && !array_filter($constraints, $isNoConstraint))) {
+        if (array_filter($constraints, $isNoConstraint)) {
             throw InvalidArgumentException::create(1, sprintf('array of %s', Constraint::class));
         }
 
@@ -167,12 +167,7 @@ protected function additionalFailureDescription($other): string
         $table->setHeaders([ 'Key', 'Value', 'Constraint' ]);
 
         foreach ($this->constraints as $key => $constraint) {
-            $valueExists = false;
-            if (is_array($other)) {
-                $valueExists = array_key_exists($key, $other);
-            } elseif ($other instanceof ArrayAccess) {
-                $valueExists = $other->offsetExists($key);
-            }
+            $valueExists = is_array($other) ? array_key_exists($key, $other) : $other->offsetExists($key);
 
             $table->addRow([
                 $this->exporter()->export($key),

src/Constraint/SequentialArray.php

@@ -53,7 +53,7 @@ class SequentialArray extends Constraint
     /** @var int|null */
     protected $maxItems;
 
-    /** @var Constraint */
+    /** @var Constraint|null */
     protected $constraint;
 
     /**
@@ -106,7 +106,7 @@ public function toString(): string
                 }
             } elseif ($this->minItems) {
                 $text .= ' with ≥ ' . $this->minItems . ' items';
-            } else {
+            } elseif ($this->maxItems) {
                 $text .= ' with ≤ ' . $this->maxItems . ' items';
             }