[protobuf] fix generation of enums with UNSPECIFIED values

[leogomes-1a]

When adding a default UNSPECIFIED value (introduced here), if an UNSPECIFIED enum value is already present the generator currently duplicates the UNSPECIFIED enum value, generating broken code. This PR fix that by simply checking if an UNSPECIFIED enum value is already present and only adding one if it's not yet defined.

@wing328 and @lucy66hw FYI :)

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[wing328]

thanks for the pr

can we keep the same yaml format before to show the example?

[leogomes-1a]

Hey, sure! Thanks for the comment :) I will fix the formatting, I messed it up and it makes it harder to review. I will fix it today!

[leogomes-1a]

Hi @wing328 It should be fixed now :)

[wing328]

minor suggestion: remove this blank line

[leogomes-1a]

It should be all good now @wing328, thank you!

[wing328]

thanks. let's give it a try