make plotBaselineDensity work without warning for single-sequence input

[ssnn]

re commit f3f676d

Make plotBaselineDensity work without warnings for single-sequence input, or change the test is the warning is expected.

expect_silent(plotBaselineDensity(groupBaselineMulti, idColumn="SUBJ", groupColumn="SUBJ"))

expect_silent(plotBaselineDensity(groupBaselineSingle, idColumn="SUBJ", groupColumn="SUBJ"))

[ssnn]

188c911