early returns means trace at a particular step may not get triggered

[yjunechoe]

Step 2 returns empty df if data is null, so breaks early here:

> ggtrace(
  ggplot2:::Layer$map_statistic,
  seq_len(length(ggbody(ggplot2:::Layer$map_statistic))),
  quote(1 + 1)
)
> ggplot()
Triggering trace on ggplot2:::Layer$map_statistic

[Step 1]> 1 + 1
[1] 2

[Step 2]> 1 + 1
[1] 2

Call `last_ggtrace()` to get the trace dump.
Untracing ggplot2:::Layer$map_statistic on exit.

Step 8 returns data if there are no calculated or staged aesthetics, so it returns early for StatIdentity as well

> ggtrace(
  ggplot2:::Layer$map_statistic,
  seq_len(length(ggbody(ggplot2:::Layer$map_statistic))),
  quote(1 + 1)
)
> ggplot(mtcars, aes(mpg, hp)) + geom_point()
Triggering trace on ggplot2:::Layer$map_statistic

[Step 1]> 1 + 1
[1] 2

[Step 2]> 1 + 1
[1] 2

[Step 3]> 1 + 1
[1] 2

[Step 4]> 1 + 1
[1] 2

[Step 5]> 1 + 1
[1] 2

[Step 6]> 1 + 1
[1] 2

[Step 7]> 1 + 1
[1] 2

[Step 8]> 1 + 1
[1] 2

Call `last_ggtrace()` to get the trace dump.
Untracing ggplot2:::Layer$map_statistic on exit.

[yjunechoe]

worth noting that this was discovered by chance while working on #19 - Layer$map_statistic returns early if there are no mapped aes