fix: comparison for empty/nil, fixes #321

src/drop/null-drop.ts

@@ -1,11 +1,10 @@
 import { Drop } from './drop'
 import { Comparable } from './comparable'
 import { isNil, toValue } from '../util/underscore'
-import { BlankDrop } from '../drop/blank-drop'
 
 export class NullDrop extends Drop implements Comparable {
   public equals (value: any) {
-    return isNil(toValue(value)) || value instanceof BlankDrop
+    return isNil(toValue(value))
   }
   public gt () {
     return false