powercap: dtpm_cpu: Fix NULL pointer dereference in get_pd_power_uw()

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

powercap: dtpm_cpu: Fix NULL pointer dereference in get_pd_power_uw()

The get_pd_power_uw() function can crash with a NULL pointer dereference when em_cpu_get() returns NULL. This occurs when a CPU becomes impossible during runtime, causing get_cpu_device() to return NULL, which propagates through em_cpu_get() and leads to a crash when em_span_cpus() dereferences the NULL pointer.

Add a NULL check after em_cpu_get() and return 0 if unavailable, matching the existing fallback behavior in __dtpm_cpu_setup().

[ rjw: Drop an excess empty code line ]