Avoid exceptions accessing nonexistent etags in RedisGrainStorage

[hendrikdevloed]

This is a Redis-only solution for the issue which was reported to affect both ADO.NET and Redis. #3516 (comment)

The workaround so far was to avoid clearing nonexistent state, but that put the burden on the user of the persistence layer. In case of a third-party library using the persistence layer, this would require patching it.

Cause

The Redis built-in LUA engine has a peculiarity that it properly returns "nil" for nonexistent results when accessed e.g. through the EVAL "script" command, but within the script it translate these "nil" to "false", which makes "==nil" checks behave in a non-intuitive manner.
redis/redis#2721

Solution

As recommended by @ReubenBond, the C# code that builds up the argument list and calls the LUA script was left untouched. The empty string as a proxy for a null was kept. Next to that, the LUA nil behaviour was taken into account to check something for its absence: (not something or something='') instead of "==nil" which doesn't work as expected intuitively.

Next to this solution, some equivalent fix for ADO.NET will be required to address further #3516

Microsoft Reviewers: Open in CodeFlow

[ReubenBond]

Thank you, @hendrikdevloed!