fix: Avoid nil pointer dereferences in faucet Github fetcher

[ajnavarro]

We got some unexpected empty fields. Covering all the cases where this might happen.

[Gno2D2]

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[codecov]

Codecov Report

❌ Patch coverage is 20.58824% with 27 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
contribs/gnofaucet/github/api.go	0.00%	12 Missing and 3 partials ⚠️
contribs/gnofaucet/github/fetcher.go	36.84%	8 Missing and 4 partials ⚠️

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