perf stat: Drop metric-unit if unit is NULL

Avoid cases like:
```
$ perf stat -a -M topdownl1 -j -I 1000
...
{"interval" : 11.127757275, "counter-value" : "85715898.000000", "unit" : "", "event" : "IDQ.MITE_UOPS", "event-runtime" : 988376123, "pcnt-running" : 100.00, "metric-value" : "0.000000", "metric-unit" : "(null)"}
...
```

If there is no unit then drop the metric-value too as:
Suggested-by: Tim Chen <[email protected]>

Signed-off-by: Ian Rogers <[email protected]>
Cc: Yicong Yang <[email protected]>
Cc: Weilin Wang <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: James Clark <[email protected]>
Cc: Mike Leach <[email protected]>
Cc: Leo Yan <[email protected]>
Cc: Sumanth Korikkar <[email protected]>
Cc: Thomas Richter <[email protected]>
Cc: Tim Chen <[email protected]>
Cc: John Garry <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Namhyung Kim <[email protected]>

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