As the subject line says, what does the P value of limma with coef=NULL for topTable indicate?

It does an ANOVA-like F-test using all coefficients, so basically testing for any difference across the set of groups in the design.

If you type help(topTable), the help page says:

If coef is left NULL, then all the coefficients or contrasts in the fitted model are used, except that any coefficient named (Intercept) will be removed.