Small performance improvements (nodejs#2044)

* perf: improve null and undefined checks

* perf: improve formdata delete operation

lib/fetch/formdata.js

@@ -61,14 +61,7 @@ class FormData {
 
     // The delete(name) method steps are to remove all entries whose name
     // is name from this’s entry list.
-    const next = []
-    for (const entry of this[kState]) {
-      if (entry.name !== name) {
-        next.push(entry)
-      }
-    }
-
-    this[kState] = next
+    this[kState] = this[kState].filter(entry => entry.name !== name)
   }
 
   get (name) {

lib/fetch/request.js

@@ -128,7 +128,7 @@ class Request {
     }
 
     // 10. If init["window"] exists and is non-null, then throw a TypeError.
-    if (init.window !== undefined && init.window != null) {
+    if (init.window != null) {
       throw new TypeError(`'window' option '${window}' must be null`)
     }
 
@@ -427,7 +427,7 @@ class Request {
     // non-null, and request’s method is `GET` or `HEAD`, then throw a
     // TypeError.
     if (
-      ((init.body !== undefined && init.body != null) || inputBody != null) &&
+      (init.body != null || inputBody != null) &&
       (request.method === 'GET' || request.method === 'HEAD')
     ) {
       throw new TypeError('Request with GET/HEAD method cannot have body.')
@@ -437,7 +437,7 @@ class Request {
     let initBody = null
 
     // 36. If init["body"] exists and is non-null, then:
-    if (init.body !== undefined && init.body != null) {
+    if (init.body != null) {
       // 1. Let Content-Type be null.
       // 2. Set initBody and Content-Type to the result of extracting
       // init["body"], with keepalive set to request’s keepalive.