Add a required paragraph break.

Add a required paragraph break.

This is necessary, for example, if a standard paragraph is nested
in an environment and the environment does not end immediately after.
To be strictly correct, the layout of the following paragraph should
be compared to the layout of the nesting environment, otherwise, if
they are not the same, an empty line is nevertheless output. However,
this is harmless because an "\end{layout}" tag immediately follows.