fix(rigging): select similar bones with empty bone name property

[GixoXYZ]

System Information
Operating system: All

Blender Version
Broken: version: All
Worked: -

GBH Tool Version
Broken: version: 2.2.0.alpha.3
Worked: -

Short description of error
Using gbh.select_similar_bones( ) with empty prefs.arm_name_bone property produces ValueError.

Exact steps for others to reproduce the error
Create a rig for a hair object, clean the prefs.arm_name_bone property, change to pose mode then use gbh.select_similar_bones( ).
Produced error:

File: "/operators/rig_ops.py", line 399
in execute
    name_pattern = bone_name.split(gbh_rig.arm_name_bone)[-1]
ValueError: empty separator