Is it true that ...

If there were no trees on my measure plot, I could measure the next count plot instead?

No. Just do without the measurements. Anytime you start changing what you are doing depending upon the results you are getting it is likely to be a mistake. You should not change to a prism that will "get" trees by reaching out further and you should not decide to measure the next plot because you feel that you need more trees.

If you were to do this, you would end up measuring more plots surrounding "zero" plots, and therefore produce a bias in the results. Would this be a big deal? Hard to tell, but why borrow trouble. What makes you think that you needed that many trees to begin with? It never pays to introduce bias just to humor a rule of thumb that is probably wrong anyway.

In the end, look at your results to find out if you needed more trees. If so, you can always go out and get more. The same is true of count plots. Most cruisers put in a few extra plots anyway, and if you get the standard error you wanted you are OK.