if [ -e foo ]
then
  # TODO: handle this
fi

if [ -e foo ]
then
  # TODO: handle this
  true
fi

# Or use the no-op colon operator ":"
if [ -e foo ]
then
  # TODO: handle this
  :
fi

Shells do not allow empty then clauses. They need at least one command (and comments are not commands).

If you want a then clause that does nothing, use a dummy command like true.

None.