[8.x] Fix morphTo attempting to map an empty string morph type to an instance

src/Illuminate/Database/Eloquent/Concerns/HasRelationships.php

@@ -256,7 +256,7 @@ public function morphTo($name = null, $type = null, $id = null, $ownerKey = null
         // If the type value is null it is probably safe to assume we're eager loading
         // the relationship. In this case we'll just pass in a dummy query where we
         // need to remove any eager loads that may already be defined on a model.
-        return is_null($class = $this->getAttributeFromArray($type))
+        return is_null($class = $this->getAttributeFromArray($type)) || $class === ''
                     ? $this->morphEagerTo($name, $type, $id, $ownerKey)
                     : $this->morphInstanceTo($class, $name, $type, $id, $ownerKey);
     }

tests/Database/DatabaseEloquentMorphToTest.php

@@ -100,6 +100,16 @@ public function testMorphToWithZeroMorphType()
         $parent->relation();
     }
 
+    public function testMorphToWithEmptyStringMorphType()
+    {
+        $parent = $this->getMockBuilder(EloquentMorphToModelStub::class)->onlyMethods(['getAttributeFromArray', 'morphEagerTo', 'morphInstanceTo'])->getMock();
+        $parent->method('getAttributeFromArray')->with('relation_type')->willReturn('');
+        $parent->expects($this->once())->method('morphEagerTo');
+        $parent->expects($this->never())->method('morphInstanceTo');
+
+        $parent->relation();
+    }
+
     public function testMorphToWithSpecifiedClassDefault()
     {
         $parent = new EloquentMorphToModelStub;