fix: Check for empty spaces and illegal characters in JSON body of error responses

[Lokideos]

Resolves #1777

I noticed that sometimes GitHub returns " " string (string with one empty space) or "<illegal-character>{}" in error responses, which leads to Json::ParseError exceptions.
If we #strip the body and check that it starts with {, we will avoid errors down the road.
WDYT?

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Before the change?

• We pass the whole error response body to Sawyer::Agent.serializer#decode, which leads to errors when the body looks like " ".

After the change?

• We strip body of empty spaces before passing it to Sawyer::Agent.serializer#decode to avoid Json::ParseError exceptions.

Pull request checklist

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

No

Please see our docs on breaking changes to help!

• Yes
• No

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[github-actions]

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