An earlier commit 7b35436a introduced an encode call designed to
support Python 3.x when passing the module name to `bcc_symcache_resolve_name`.
This breaks current code because the module may be None when
resolving kernel symbols, e.g. using the `BPF.ksymname` API.

This commit fixes the regression and introduces a test for `ksym`
and `ksymname` that would have caught it in the first place.

Resolves #1054.