Fix architecture.bits tab completion

MSP-8986

architectures.bits tab completion failed because it was trying to shell
escape `nil`, which came from the architectures that don't have bits,
like Ruby and Python.  Exclude returning NULL values for tab completion
since the search syntax can't differentiate between blank '' and an
actual nil since it's all '' when parsed.
Fix architectures operators

MSP-8986

The set operators for architectures raised an exception when no value
was given instead of showing a validation error because
`Metasploit::Model::Architecture::ClassMethods#set`, which is used to
find to find and display the set of allowed values was including the
`nil` value for bits, endianness, and family and nil cannot sort with
other types.  The `nil`s were removed in metasploit-model
0.19.3.tab-completion.