package_rpm: Clean up unset runtime package variable handling
If the R* runtime package variables are unset it could cause trackbacks. There
were some fallbacks already, clean this up to handle consistently.
The code was expecting strings but setting defaults of empty lists
which silently were converted to strings by the "or" statements which
was a nightmare to understand or alter.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
