Fix user stats showing -Inf/NaN for vote metrics when user has no votes

[Copilot]

R's max() and mean() return -Inf and NaN on empty vectors, causing those values to surface in the user stats table when a user has not yet cast any votes.

Changes

• R/mod_userstats.R: Initialize average_votes_per_session and max_votes_per_session to NA, computing them only when nrow(session_counts_df) > 0 — consistent with the existing guard pattern already used for session_times and time_vals in the same function.

• tests/testthat/test-user-stats-module.R: Add a test covering the zero-votes case, asserting that neither -Inf nor NaN appear in the stats output for these metrics.

# Before
max_votes_per_session = max(session_counts_df$images_voted),   # → -Inf when empty

# After
max_votes_per_session <- NA
if (nrow(session_counts_df) > 0) {
  max_votes_per_session <- max(session_counts_df$images_voted)
}