fix: pass drops directly to filters/tags

src/drop/null-drop.ts

@@ -1,11 +1,11 @@
 import { Drop } from './drop'
 import { IComparable } from './icomparable'
-import { isNil } from '../util/underscore'
+import { isNil, toValue } from '../util/underscore'
 import { BlankDrop } from '../drop/blank-drop'
 
 export class NullDrop extends Drop implements IComparable {
   equals (value: any) {
-    return isNil(value instanceof Drop ? value.valueOf() : value) || value instanceof BlankDrop
+    return isNil(toValue(value)) || value instanceof BlankDrop
   }
   gt () {
     return false