Support null eTag

[artursouza]

Expected Behavior

Comply with new proto for etag as per https://github.com/dapr/dapr/pull/2666/files

Actual Behavior

eTag is still handled as a simple string.

Steps to Reproduce the Problem

Use master version of daprd and use any API that uses etag.

Release Note

RELEASE NOTE: FIXED Support for null and empty etag.