update set nullable

src/Driver/Traits/SQL.php

@@ -746,7 +746,11 @@ trait SQL
                 return $this;
             }
         }
-        $val = $this->handleFieldAlias($field) ."='". $this->setQuote($value) . "'";
+        if($value == null){
+            $val = $this->handleFieldAlias($field) ." = null";
+        }else{
+            $val = $this->handleFieldAlias($field) ."='". $this->setQuote($value) . "'";
+        }
         $this->sets = $this->sets != null ? $this->sets . ', '. $val : $val;
         return $this;
     }