Lua: luaL_checkstring does not return NULL

Removed all checks for NULL strings to cleanup the code.

Change-Id: Ia890cd9b206296f586e85214f07765f14984580b
Reviewed-on: https://code.wireshark.org/review/4632
Tested-by: Stig Bjørlykke <stig@bjorlykke.org>
Reviewed-by: Stig Bjørlykke <stig@bjorlykke.org>