fix: pass drops directly to filters/tags

src/drop/blank-drop.ts

@@ -1,11 +1,10 @@
-import { isNil, isString } from '../util/underscore'
-import { Drop } from '../drop/drop'
+import { isNil, isString, toValue } from '../util/underscore'
 import { EmptyDrop } from '../drop/empty-drop'
 
 export class BlankDrop extends EmptyDrop {
   equals (value: any) {
     if (value === false) return true
-    if (isNil(value instanceof Drop ? value.valueOf() : value)) return true
+    if (isNil(toValue(value))) return true
     if (isString(value)) return /^\s*$/.test(value)
     return super.equals(value)
   }

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