Fix ggplot2 ‘Groups with fewer than two datapoints’ warning in mcmc_nuts_treedepth()

[BjarkeHautop]

Currently, the following gives a warning (taken from test-mcmc-nuts.R)

> if (requireNamespace("rstanarm", quietly = TRUE)) {
+     ITER <- 1000
+     CHAINS <- 3
+     fit <- rstanarm::stan_glm(mpg ~ wt + am, data = mtcars,
+                               iter = ITER, chains = CHAINS,
+                               refresh = 0)
+     np <- nuts_params(fit)
+     lp <- log_posterior(fit)
+ }
> 
> mcmc_nuts_treedepth(np, lp, chain = CHAINS)
Warning messages:
1: Groups with fewer than two datapoints have been dropped.
ℹ Set `drop = FALSE` to consider such groups for position adjustment purposes. 
2: Groups with fewer than two datapoints have been dropped.
ℹ Set `drop = FALSE` to consider such groups for position adjustment purposes.

This PR fixes the issue by filtering that there are at least 2 observations before calling geom_violin, so the dropping doesn't happen implicitly with a warning.

[jgabry]

Great, thank you very much! Tests are running now.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.62%. Comparing base (5a5b1e5) to head (cfb5055).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #412   +/-   ##
=======================================
  Coverage   98.61%   98.62%           
=======================================
  Files          35       35           
  Lines        5792     5798    +6     
=======================================
+ Hits         5712     5718    +6     
  Misses         80       80           

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[jgabry]

Changes look good and all tests are passing. Merging now. Thanks again!