drm/panthor: skip zero-sized firmware sections

In the Linux kernel, the following vulnerability has been resolved:

drm/panthor: skip zero-sized firmware sections

panthor_fw_load_section_entry() skips BO creation when the firmware section
VA range is empty. If such a section is added to the firmware section list,
section->mem is left as NULL.

Later reload and unplug paths iterate over all firmware sections and
dereference section->mem, which can lead to a NULL pointer dereference.

Zero-sized firmware sections are valid, so accept them as no-op entries but
skip adding them to the section list.