Make pascal_language::print_type handle varstring==nullptr

PR gdb/31524 points out a crash when pascal_language::print_type is
called with varstring==nullptr.  This crash is a regression arising
from the printf/pager rewrite -- that indirectly removed a NULL check
from gdb's "puts".

This patch instead fixes the problem by adding a check to print_type.
Passing nullptr here seems to be expected in other places (e.g., there
is a call to type_print like this in expprint.c), and other
implementations of this method (or related helpers) explicitly check
for NULL.

I didn't write a test case for this because it seemed like overkill
for a Pascal bug that only occurs with -i=mi.  However, if you want
one, let me know and I will do it.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31524
Approved-By: John Baldwin <jhb@FreeBSD.org>

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