Fix candidates_formatted not found in print.mo_uncertainties()

When there is only one match (candidates field is empty), the "Also
matched:" block must be skipped. Wrapping it in if (candidates != "")
prevents the error that arose after #288 narrowed filtr to one entry.

https://claude.ai/code/session_01VH4Ju4Xq9aW1AHuoVbjGEo

R/mo.R

@@ -1023,6 +1023,7 @@ print.mo_uncertainties <- function(x, n = 10, ...) {
       message_(out2, as_note = FALSE)
     }
 
+    if (x[i, ]$candidates != "") {
       other_matches <- paste0(
         "Also matched: ",
         vector_and(
@@ -1035,6 +1036,7 @@ print.mo_uncertainties <- function(x, n = 10, ...) {
       )
       message_(other_matches, as_note = FALSE)
     }
+  }
 
   if (isTRUE(any_maxed_out)) {
     cat("\n")