lminiz: return nil on deflate/inflate failure

[Bilal2453]

When miniz returns NULL on tdefl_compress_mem_to_heap and tinfl_decompress_mem_to_heap signaling an error, lminiz will still return a string to the end user, an empty string in this case. This pull changes this behavior to return back a nil value plus an error message to be consistent with the rest of the API.

This change may not be necessarily wanted, as an empty string should still be valid for an error indication, as it seems miniz refuse to inflate a ZLib-ed empty string (i.e. an empty string with zlib header only, e.g. miniz.deflate("")).