Ensure we check for the key to exist before attempting to read it

[sarahhenkens]

This PR fixed #165, ensuring we check for the key existence before we pass it into json.Marshal.

data[secretKey] returns nil and json.Marshal(nil) hides the fact that the secretKey doesn't exist.

➜ make test 
gotestsum --format=short-verbose 
PASS internal/version.TestGetVersion (0.00s)
PASS internal/version
PASS internal/config.TestParseParametersFromYaml (0.00s)
PASS internal/config.TestParseParameters (0.00s)
PASS internal/config.TestParseConfig (0.00s)
PASS internal/config.TestParseConfig_Errors (0.00s)
PASS internal/config.TestValidateConfig (0.00s)
PASS internal/config
PASS TestListen (0.00s)
PASS .
PASS internal/provider.TestEnsureV1Prefix (0.00s)
PASS internal/provider.TestGenerateRequest (0.00s)
PASS internal/provider.TestKeyFromData (0.00s)
PASS internal/provider.TestKeyFromDataMissingKey (0.00s)
PASS internal/provider.TestHandleMountRequest (0.12s)
PASS internal/provider
PASS internal/client.TestNew (1.81s)
PASS internal/client.TestConfigPrecedence (0.00s)
PASS internal/client.TestNew_Error (0.00s)
PASS internal/client
EMPTY internal/server

DONE 15 tests in 6.840s

[tomhjp]

This looks great, thanks for including so much detail and tests upfront! I just have one suggestion to include more detail in the error message, and please could you also add a short entry to the changelog? Then I think this will be ready to merge.

[tomhjp]

Hope you don't mind I made 3 characters worth of nit edits just to save another round of review :)

Thanks for contributing this! It looks great 👌