Only populate secondary endpoints if the config is set

[KenGordon]

First change to support Azure port. It came about as a result of the Azure KMS implementation being different - there is only ever one KMS and so there is never a SECONDARY_COORDINATOR_PRIVATE_KEY_ENDPOINT.

This change does two main things:

1. Modifies TrustedServersConfigClient::HasParameter() to return false when parameter is present in config_entries_map but has a default empty value.

   • This is because all flags passed into the constructor of TrustedServersConfigClient are added to the map with the empty value
   • So if SECONDARY_COORDINATOR_PRIVATE_KEY_ENDPOINT isn't specified, TrustedServersConfigClient::HasParameter(SECONDARY_COORDINATOR_PRIVATE_KEY_ENDPOINT) incorrectly returns true

2. When initialising PrivateKeyFetcherFactory, reorganises the logic so that the secondary endpoint is only included when SECONDARY_COORDINATOR_PRIVATE_KEY_ENDPOINT is set

These two changes mean that when combining primary and secondary endpoints into a single vector in the data plane, we don't get a secondary endpoint with a blank URL

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