Handle missing plot_legend1 in data_transformation_plot else condition

[pdiakumis]

The data_transformation_plot chunk has an if-else condition that handles the filter param. The plot_legend1 variable is defined and used in the if part, but it's also used in the else part. This results in:

processing file: rnasum.Rmd
  |...                        |  11%                                            [data_transformation_plot]                 [goi_summary]                             
Quitting from lines 573-766 [data_transformation_plot] (rnasum.Rmd)
Error in `as.graphicsAnnot()`:
! object 'plot_legend1' not found
Backtrace:
 1. graphics::legend(...)
 2. grDevices::as.graphicsAnnot(legend)
                                                                                                                                     
Execution halted