perf mem/c2c: Avoid printing empty lines for unsupported events

The 'perf mem' and 'perf c2c' tools can be used with 3 different events:
load, store and combined load-store. Some architectures might support
only partial set of events in which case, perf prints an empty line for
unsupported events.  Avoid that.

Ex, AMD Zen cpus supports only combined load-store event and does not
support individual load and store event.

Before patch:
  $ perf mem record -e list

  mem-ldst     : available
  $

After patch:

  $ perf mem record -e list
  mem-ldst     : available
  $

Signed-off-by: Ravi Bangoria <[email protected]>
Acked-by: Jiri Olsa <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Ali Saidi <[email protected]>
Cc: Ananth Narayan <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Ian Rogers <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Joe Mario <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Kim Phillips <[email protected]>
Cc: Leo Yan <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Sandipan Das <[email protected]>
Cc: Santosh Shukla <[email protected]>
Cc: Stephane Eranian <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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