fix: updated isNode check (protobufjs#1221) (protobufjs#1363)

* Fixes protobufjs#1221 when used within SSR.
`process` is a global variable since node [0.1.7](https://nodejs.org/api/globals.html#globals_process) and versions since [0.2.0](https://nodejs.org/api/process.html#process_process_versions).

Co-authored-by: Alexander Fenster <fenster@google.com>

src/util/minimal.js

@@ -52,7 +52,7 @@ util.emptyObject = Object.freeze ? Object.freeze({}) : /* istanbul ignore next *
  * @type {boolean}
  * @const
  */
-util.isNode = Boolean(util.global.process && util.global.process.versions && util.global.process.versions.node);
+util.isNode = Boolean(process && process.versions && process.versions.node);
 
 /**
  * Tests if the specified value is an integer.