fs: simplify fs.promises warning logic

fs.promises is lazy loaded. Instead of using a seaparate
Boolean flag to track whether or not it has been loaded, just
inspect the state of the lazy loaded module itself.

PR-URL: #24788
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Masashi Hirano <shisama07@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

Author: cjihrig

lib/fs.js

@@ -86,12 +86,11 @@ const {
   validateUint32
 } = require('internal/validators');
 
-let promisesWarn = true;
 let truncateWarn = true;
 let fs;
 
 // Lazy loaded
-let promises;
+let promises = null;
 let watchers;
 let ReadFileContext;
 
@@ -1837,9 +1836,8 @@ Object.defineProperties(fs, {
     configurable: true,
     enumerable: false,
     get() {
-      if (promisesWarn) {
+      if (promises === null) {
         promises = require('internal/fs/promises');
-        promisesWarn = false;
         process.emitWarning('The fs.promises API is experimental',
                             'ExperimentalWarning');
       }