fix: comparison for empty/nil, fixes #321

src/drop/empty-drop.ts

@@ -1,9 +1,11 @@
 import { Drop } from './drop'
 import { Comparable } from './comparable'
-import { isObject, isString, isArray } from '../util/underscore'
+import { isObject, isString, isArray, toValue } from '../util/underscore'
 
 export class EmptyDrop extends Drop implements Comparable {
   public equals (value: any) {
+    if (value instanceof EmptyDrop) return false
+    value = toValue(value)
     if (isString(value) || isArray(value)) return value.length === 0
     if (isObject(value)) return Object.keys(value).length === 0
     return false

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