fix when unknown type is found in foreach expression

fixtures/completion/foreach.php

@@ -35,3 +35,6 @@ foreach ($array3 as $key => $value) {
 foreach ($bar->test() as $value) {
     $
 }
+
+foreach ($unknownArray as $unknown) {
+    $unkno

src/DefinitionResolver.php

@@ -1122,6 +1122,7 @@ class DefinitionResolver
             if ($collectionType instanceof Types\Array_) {
                 return $collectionType->getValueType();
             }
+            return new Types\Mixed_();
         }
 
         // PROPERTIES, CONSTS, CLASS CONSTS, ASSIGNMENT EXPRESSIONS