perf(grpc-js): avoid allocations for empty CallCredentials on the RPC hot path

[olavloite]

Previously, every RPC executed through LoadBalancingCall composed per-call and subchannel CallCredentials and called generateMetadata(), even when both sides were EmptyCallCredentials. In addition:

• Composing any CallCredentials with EmptyCallCredentials on the right-hand side wrapped them in a new ComposedCallCredentials instance.
• InsecureChannelCredentialsImpl allocated a new EmptyCallCredentials instance on every getCallCredentials() call when no call credentials were provided.
• EmptyCallCredentials.generateMetadata() allocated a new Metadata instance and Promise on every RPC only to merge an empty map.

This change:

• Treats EmptyCallCredentials as a two-sided identity in compose(), returning this directly when composing with an empty instance.
• Evaluates the fallback CallCredentials.createEmpty() once per connector in InsecureChannelCredentialsImpl._createSecureConnector().
• Short-circuits metadata generation and merging in LoadBalancingCall when the composed CallCredentials is empty, reusing a pre-resolved Promise.