buffer: increase coverage by removing dead code

buffer.js:L196 `if (value == null)` guarantees `obj != null`
so L406+L418 are unnecessary.

PR-URL: #15100
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/buffer.js

@@ -404,17 +404,15 @@ function fromObject(obj) {
     return b;
   }
 
-  if (obj != null) {
-    if (obj.length !== undefined || isAnyArrayBuffer(obj.buffer)) {
-      if (typeof obj.length !== 'number' || obj.length !== obj.length) {
-        return new FastBuffer();
-      }
-      return fromArrayLike(obj);
+  if (obj.length !== undefined || isAnyArrayBuffer(obj.buffer)) {
+    if (typeof obj.length !== 'number' || obj.length !== obj.length) {
+      return new FastBuffer();
     }
+    return fromArrayLike(obj);
+  }
 
-    if (obj.type === 'Buffer' && Array.isArray(obj.data)) {
-      return fromArrayLike(obj.data);
-    }
+  if (obj.type === 'Buffer' && Array.isArray(obj.data)) {
+    return fromArrayLike(obj.data);
   }
 }