perf: Return empty callchain instead of NULL

It simplifies the code a bit, because we dump the callchain
Link: http://lkml.kernel.org/n/[email protected]
even if it's empty. With 'empty' callchain we can remove
all the NULL-checking code paths.

Original-patch-from: Peter Zijlstra <[email protected]>
Signed-off-by: Jiri Olsa <[email protected]>
Acked-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Namhyung Kim <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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