perf(_comp_delimited): add "$prefix" only when it is non-empty

Author: akinomyoga

bash_completion

@@ -1352,10 +1352,12 @@ _comp_delimited()
     # `show-all-if-ambiguous` on, but even that has cases where it fails
     # and the last separator including everything before it is lost.
     # https://github.com/scop/bash-completion/pull/913#issuecomment-1490140309
-    local i
-    for i in "${!COMPREPLY[@]}"; do
-        COMPREPLY[i]="$prefix${COMPREPLY[i]}"
-    done
+    if [[ $prefix ]]; then
+        local i
+        for i in "${!COMPREPLY[@]}"; do
+            COMPREPLY[i]="$prefix${COMPREPLY[i]}"
+        done
+    fi
 
     [[ $delimiter != : ]] || _comp_ltrim_colon_completions "$cur"
 }