Correct handling of empty reactions.

Signed-off-by: Egon Willighagen <egonw@users.sourceforge.net>
Allow SMARTS expressions of the form [h0] and [!h0] analogous to [H0] and [!H0]. Previously this was not possible to null handling semantics.

Signed-off-by: Egon Willighagen <egonw@users.sourceforge.net>