perf/x86/rapl: Fix "Using plain integer as NULL pointer" Sparse warning

Change 0 to NULL when initializing the test field of perf_msr structs to
avoid the following sparse warnings:

  make C=2 arch/x86/events/rapl.o

  CHECK   arch/x86/events/rapl.c
  ...
  arch/x86/events/rapl.c:540:59: warning: Using plain integer as NULL pointer
  arch/x86/events/rapl.c:542:59: warning: Using plain integer as NULL pointer
  arch/x86/events/rapl.c:543:59: warning: Using plain integer as NULL pointer
  arch/x86/events/rapl.c:544:59: warning: Using plain integer as NULL pointer

Signed-off-by: David Reaver <me@davidreaver.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20230801155651.108076-1-me@davidreaver.com