Fix separator getting added to variables_prefix when empty

[rajaths010494]

Describe the bug
azure key vault adds a - between the key prefix and the secret name when it reads it from the azure key vault. It does this even when the variables prefix is "" so secrets end up named -my-secret instead of my-secret.

To Reproduce
Follow the instructions to set up an azure key vault integrated install with a variables_prefix of "".
Create a secret named -my-var in the azure key vault.
Use the example dag provided in the docs with Variable.get("my-var")
You will see that it fetches the variable because it is incorrectly mapping my-var to -my-var instead of my-var.

Expected Behaviour
connections_prefix and variables_prefix don't add a leading - when they are set to "".

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[rajaths010494]

tests/providers/google/cloud/utils/test_credentials_provider.py::TestGetGcpCredentialsAndProjectId::test_disable_logging: Failed: DID NOT RAISE <class 'AssertionError'> is failing but i haven't changed anything in Google cloud.