Make the handling of empty .tab files a bit stricter
Make the handling of empty .tab files a bit stricter

Previously, if the first or second lines were empty, the whole of the
file was skipped (for files loaded with optional=True), even if more
lines were present.

Instead, only skip the file if it contains 0 or 1 lines.  Validate the
headings if they are present.

bmaluenda/switch