Fix empty value state decryption panic

[yaron2]

As reported on Discord, trying to decrypt an empty value results in the following panic:

panic: runtime error: slice bounds out of range [1:0]

goroutine 164 [running]:
github.com/dapr/dapr/pkg/encryption.TryDecryptValue({0xc000b8f320, 0xb}, {0x0, 0x0, 0x0})
        D:/a/dapr/dapr/pkg/encryption/state.go:64 +0x2f8
github.com/dapr/dapr/pkg/grpc.(api).GetState(0xc00149a000, {0x6578618, 0xc0013d1230}, 0xc0005fb860)
        D:/a/dapr/dapr/pkg/grpc/api.go:843 +0x4bf

[codecov]

Codecov Report

Merging #6321 (fdfaf37) into master (546ca51) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #6321   +/-   ##
=======================================
  Coverage   64.47%   64.48%           
=======================================
  Files         189      189           
  Lines       18923    18925    +2     
=======================================
+ Hits        12200    12203    +3     
  Misses       5756     5756           
+ Partials      967      966    -1     

Impacted Files	Coverage Δ
pkg/encryption/state.go	84.00% <100.00%> (+1.39%)	⬆️

... and 3 files with indirect coverage changes