fix(filter): handle nil argument for replace_last (#177)

When `replace_last` is called with a `nil` value for the string to be replaced, it should append the replacement string to the input.

This was not happening, and the filter was returning the input string unchanged.

This commit fixes this by checking if the string to be replaced is empty after being converted to a string, and if so, appending the replacement.

A doctest and an integration test have been added to cover this case.

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

Author: google-labs-jules[bot]

lib/solid/standard_filter.ex

@@ -705,25 +705,31 @@ defmodule Solid.StandardFilter do
   "foo"
   iex> Solid.StandardFilter.replace_last("foo", "f", "b")
   "boo"
+  iex> Solid.StandardFilter.replace_last("Take my protein", nil, "#")
+  "Take my protein#"
   """
   @spec replace_last(String.t(), String.t(), String.t()) :: String.t()
   def replace_last(input, string, replacement) do
     input = to_str(input)
-    string = to_str(string)
+    string_arg = to_str(string)
     replacement = to_str(replacement)
 
-    case last_index(input, string) do
+    case last_index(input, string_arg) do
       nil ->
-        input
+        if string_arg == "" do
+          input <> replacement
+        else
+          input
+        end
 
       index ->
         prefix = :binary.part(input, 0, index)
 
         suffix =
           :binary.part(
             input,
-            index + byte_size(string),
-            byte_size(input) - (index + byte_size(string))
+            index + byte_size(string_arg),
+            byte_size(input) - (index + byte_size(string_arg))
           )
 
         prefix <> replacement <> suffix

test/solid/integration/scenarios/filter/input.liquid

@@ -460,6 +460,7 @@ base64
 {{ "hello" | replace_last: "ll" }}
 {{ "hello" | replace_last: "l", "p" }}
 {{ "abab" | replace_last: "a", "c" }}
+{{ "Take my protein" | replace_last: nosuchthing, "#" }}
 
 default