showLinePlots should show empty plot with variable name when no data in mainReg is given

[orichters]

Somehow, it doesn't, leading to useless plots without data:

https://github.com/pik-piam/mip/blob/master/R/showLinePlots.R

How to reproduce something like that:

d <- dplyr::filter(quitte::quitte_example_data, variable == "Consumption", scenario == "r7552c_1p5C_Def-rem-5", region != "World")
mip::showLinePlots(d, "Consumption", mainReg = "World")