Add NilSliceAsEmpty encode option

[w1ck3dg0ph3r]

This is a proposal to add an option NilSliceAsEmpty to encode nil slices and pointers to nil slices as [] instead of null.

[tomaswarynyca]

@goccy What do you think about adding this functionality?

[goccy]

I'm concerned that enabling this option adds a branch to the runtime code path, which hurts performance.
I think it is necessary to discuss whether the function is really necessary.

[vx416]

This function is valuable for decoding protocol buffers because the slice in the gRPC response received by the client-side will be nil if the server initializes an empty slice.

[ivanjaros]

#437