readBulkString does not work properly for RESP_NIL

[SyedMa3]

readBulkString function in core/resp.go does not handle RESP_NIL correctly and times out.

This also blocks Issue #200 since test cases have to be created there.

How to reproduce

In integration tests, add a case where the expected value of a command is RESP_NIL.

Expected behavior It should return (nil)

[SyedMa3]

I have solved this issue. I will create a PR for this.