When empty data are provided to matsindf_apply(),
care must be take with the return value.
This function assembles the correct zero-row data frame or
zero-length lists.
handle_empty_data(.dat = NULL, FUN, DF, types)The appropriate return value from matsindf_apply(),
either a zero-length list or a zero-row data frame.
The .dat argument to matsindf_apply().
The FUN argument to matsindf_apply().
The assembled DF inside matsindf_apply().
The types object assembled inside matsindf_apply().