kernel: mmu: fix compilation warnings when memory address and size are 0

When both the memory base address & its size are zero, the
assertions test will be comparing an unsigned int against zero
which result in compilation warning, and will be raised to
error in Twister

Fix them with more conditional compilations.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
(cherry picked from commit 3ed8c4d6)