fix tab selection

[JanMarvin]

Previously this would select sheet one per default in certain cases. Related to #399 . Similar to #505 fixing old code parts, that were never understood, but updated and fixed incorrectly.

wb <- wb_load(file = system.file("extdata", "loadExample.xlsx", package = "openxlsx2"))
wb$get_sheet_names() # list worksheets
wb ## view object