In fact Robert's Rules of Order says, “In the absence of a quorum any business transacted (except for the procedural actions noted in the next paragraph) is null and void. But if a quorum fails to appear at a regular or properly called meeting, the inability to transact business does not detract from the fact that the society's rules requiring the meeting to be held were complied with and the meeting was convened -- even though it had to adjourn immediately.

The only action that can legally be taken in the absence of a quorum is to fix the time to which to adjourn, adjourn, recess, or take measures to obtain a quorum ....” RONR 10^th ed. p. 336, l. 25-35.

“The prohibition against transacting business in the absence of a quorum cannot be waived even by unanimous consent, and a notice cannot be validly given.” RONR 10^th ed. p. 337, l. 14-16.

Ah but it gives them an out. Later in the same paragraph it says, “Where an important opportunity would be lost unless acted upon immediately, the members present can, at their own risk, act in the emergency with the hope that their action will be ratified by a later meeting at which a quorum is present.” RONR 10^th ed. p. 337, l. 19-23.

Of course it requires that people act in good faith in declaring something an emergency in order for this to work properly.