fix(plot_serocurve): suppress fill scale warning when show_ci = FALSE

Move `scale_fill_manual()` inside an `if (show_ci)` guard so it is only
added when a CI ribbon is actually drawn. Previously the fill scale was
applied unconditionally for single-stratification plots, producing
"No shared levels found between names(values) of the manual scale and
the data's fill values" when show_ci was FALSE — which triggered an
R CMD check failure on the `serocurve-population-no-ci` test.

Co-authored-by: Samuel Hisaji Schildhauer <sschildhauer@users.noreply.github.com>

Author: github-actions[bot]

R/plot_serocurve.R

@@ -225,22 +225,26 @@ plot_serocurve <- function(
     color_vals <- c("median" = "red")
     color_labels <- c("median" = "Median")
 
-    fill_vals <- c("ci" = "red")
-    fill_labels <- c("ci" = "95% credible interval")
-
     p <- p +
       ggplot2::scale_color_manual(
         name = "",
         values = color_vals,
         labels = color_labels,
         guide = ggplot2::guide_legend(override.aes = list(shape = NA))
-      ) +
-      ggplot2::scale_fill_manual(
-        name = "",
-        values = fill_vals,
-        labels = fill_labels,
-        guide = ggplot2::guide_legend(override.aes = list(color = NA))
       )
+
+    if (show_ci) {
+      fill_vals <- c("ci" = "red")
+      fill_labels <- c("ci" = "95% credible interval")
+
+      p <- p +
+        ggplot2::scale_fill_manual(
+          name = "",
+          values = fill_vals,
+          labels = fill_labels,
+          guide = ggplot2::guide_legend(override.aes = list(color = NA))
+        )
+    }
   }
 
   # ---- Faceting ----------------------------------------------------------