(refactor): w/blist transform args shouldn't be optional

- the array passed into the transforms should be required
  - same for arrToDict function
  - they're now called only when array is defined, and I think that
    behavior is more intuitive -- it should error if the array isn't
    defined
    - (test): in internal usage, they're never undefined, so this was
      reducing coverage because it was dead code
    - if the transforms were to eventually be externally visible /
      import-able, they should definitely throw an error in that case
      too -- if external users pass undefined to it
      - and the typings change means it'll happen at design-time too
      - definitely think it's more intuitive for external users this
        way too

- (docs): use this commit for w/blist transform examples
  - or a variant of this commit -- just one where arr is required

README.md

@@ -83,7 +83,7 @@ interface ITransformArgs {
 }
 ```
 
-As an example, one may see how [whitelists](https://github.com/agilgur5/mst-persist/blob/229fd2b1b472ea6a7912a5a06fa079a65e3ba6fa/src/whitelistTransform.ts#L7-L14) and [blacklists](https://github.com/agilgur5/mst-persist/blob/229fd2b1b472ea6a7912a5a06fa079a65e3ba6fa/src/blacklistTransform.ts#L7-L14) are implemented internally as transforms.
+As an example, one may see how [whitelists](https://github.com/agilgur5/mst-persist/blob/9ba76aaf455f42e249dc855d66349351148a17da/src/whitelistTransform.ts#L7-L12) and [blacklists](https://github.com/agilgur5/mst-persist/blob/9ba76aaf455f42e249dc855d66349351148a17da/src/blacklistTransform.ts#L7-L12) are implemented internally as transforms.
 
 #### Transform Ordering
 

src/blacklistTransform.ts

@@ -1,12 +1,10 @@
 import { arrToDict } from './utils'
 import { ITransform } from './index'
 
-export function blacklistKeys (blacklist?: Array<string>): ITransform {
+export function blacklistKeys (blacklist: Array<string>): ITransform {
   const blacklistDict = arrToDict(blacklist)
 
   return {toStorage: function blacklistTransform (snapshot) {
-    if (!blacklist) { return snapshot }
-
     Object.keys(snapshot).forEach((key) => {
       if (blacklistDict[key]) { delete snapshot[key] }
     })

src/utils.ts

@@ -2,8 +2,7 @@ export type StrToAnyMap = {[key: string]: any}
 
 export type StrToBoolMap = {[key: string]: boolean}
 
-export function arrToDict (arr?: Array<string>): StrToBoolMap {
-  if (!arr) { return {} }
+export function arrToDict (arr: Array<string>): StrToBoolMap {
   return arr.reduce((dict: StrToBoolMap, elem) => {
     dict[elem] = true
     return dict

src/whitelistTransform.ts

@@ -1,12 +1,10 @@
 import { arrToDict } from './utils'
 import { ITransform } from './index'
 
-export function whitelistKeys (whitelist?: Array<string>): ITransform {
+export function whitelistKeys (whitelist: Array<string>): ITransform {
   const whitelistDict = arrToDict(whitelist)
 
   return {toStorage: function whitelistTransform (snapshot) {
-    if (!whitelist) { return snapshot }
-
     Object.keys(snapshot).forEach((key) => {
       if (!whitelistDict[key]) { delete snapshot[key] }
     })