ref(gatsby): Default release to empty string

To make gatsby work better out of the box, we set the default
for the release value to be undefined instead of an empty string.

Author: AbhiPrasad

packages/gatsby/gatsby-node.js

@@ -11,7 +11,7 @@ const sentryRelease = JSON.stringify(
     process.env.ZEIT_GITHUB_COMMIT_SHA ||
     process.env.ZEIT_GITLAB_COMMIT_SHA ||
     process.env.ZEIT_BITBUCKET_COMMIT_SHA ||
-    '',
+    undefined,
 );
 
 const sentryDsn = JSON.stringify(process.env.SENTRY_DSN || '');

packages/gatsby/test/gatsby-node.test.ts

@@ -18,7 +18,7 @@ describe('onCreateWebpackConfig', () => {
     expect(plugins.define).toHaveBeenCalledTimes(1);
     expect(plugins.define).toHaveBeenLastCalledWith({
       __SENTRY_DSN__: expect.any(String),
-      __SENTRY_RELEASE__: expect.any(String),
+      __SENTRY_RELEASE__: undefined,
     });
 
     expect(actions.setWebpackConfig).toHaveBeenCalledTimes(1);