Revert "Check if True without len calculation"
Revert "Check if True without len calculation"

This reverts commit 5d6e3bc8c7897d734b94181295b2883a3800c9ad.
Actually, it was correct to check for lenght, because a frame can have
columns, but be of length zero. Such a frame would pass the `if df`
test, but would fail later.

achabotl/pambox