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.