Skip exports if not available by CommonJS

[Danielku15]

Resolves #8855

This is a minimal fix for the problem reported in the above issue. I added a simplistic check whether the exports variable is an object to be filled. This eliminates the error without making big changes to really extend to an UMD. Proper UMD support (also in the generated code) would likely need a bigger change in code consisting of changes in the file structure, code generator (protoc), tests etc.

My primary goal is to eliminate the browser error.

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[google-cla]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[Danielku15]

@googlebot I signed it!

[Danielku15]

@lukesandberg @acozzette Any chance this will get reviewed and integrated in the near future. Otherwise I we will need likely to either patch locally the library or fully replace it with some other lib. 😢

[Danielku15]

We will try to keep our response time within 7-days but if you don’t get any response in a few days, feel free to comment on the threads to get our attention. (https://github.com/protocolbuffers/protobuf/blob/master/CONTRIBUTING.md)

@lukesandberg @acozzette
It has been now quite a while since I reported this issue and provided a PR to fix it. I saw some activities on the labels but beside that the replies are rather slient 😢 Since quite some sprints were are now shifting an open bug report on our side related to this page error. I would appreciate any feedback on whether you plan to integrate this change or whether we are better set with a different framework.

[Danielku15]

Ping @lukesandberg @acozzette Is this project still active?

[acozzette]

@Danielku15 Sorry for the delay and thank you for the fix!