Improving jank tests diagnostics

When there were 0 total frames, ProfileData generates something like:
Janky frames: 0 (nan%)

Then the test fails to parse it, and ends up with a mysterious
"Failed to parse NUM_JANKY" diag (see the bug).

Making the case 0/0 a 0%.

Bug: 77528721
Test: atest google/perf/jank/SystemUI/UbSystemUIJankTests:android.platform.systemui.tests.jank.LauncherJankTests#testOpenAllAppsContainer
Change-Id: Ib65b80dc689f7b6ee06b108114ffd7de9d739721