fix: ensure existing dependabot configs without end of file newlines get processed properly

dependabot_file.py

@@ -139,6 +139,10 @@ def build_dependabot_file(
             try:
                 if repo.file_contents(file):
                     package_managers_found[manager] = True
+                    # If the last thing in the file is not a newline,
+                    # add one before adding a new language config to the file
+                    if dependabot_file and dependabot_file[-1] != "\n":
+                        dependabot_file += "\n"
                     dependabot_file += make_dependabot_config(
                         manager, group_dependencies, indent, schedule, schedule_day
                     )

test_dependabot_file.py

@@ -108,6 +108,37 @@ def test_build_dependabot_file_with_2_space_indent_existing_config_bundler_with_
         )
         self.assertEqual(result, expected_result)
 
+    def test_build_dependabot_file_with_2_space_indent_existing_config_bundler_with_update_and_no_newline(
+        self,
+    ):
+        """Test that the dependabot.yml file is built correctly with bundler"""
+        repo = MagicMock()
+        repo.file_contents.side_effect = lambda f, filename="Gemfile": f == filename
+
+        # expected_result maintains existing ecosystem with custom configuration
+        # and adds new ecosystem
+        expected_result = """---
+version: 2
+updates:
+  - package-ecosystem: "pip"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    commit-message:
+      prefix: "chore(deps)"
+  - package-ecosystem: 'bundler'
+    directory: '/'
+    schedule:
+      interval: 'weekly'
+"""
+        existing_config = MagicMock()
+        existing_config.decoded = b'---\nversion: 2\nupdates:\n  - package-ecosystem: "pip"\n    directory: "/"\n\
+    schedule:\n      interval: "weekly"\n    commit-message:\n      prefix: "chore(deps)"'
+        result = build_dependabot_file(
+            repo, False, [], {}, existing_config, "weekly", ""
+        )
+        self.assertEqual(result, expected_result)
+
     def test_build_dependabot_file_with_weird_space_indent_existing_config_bundler_with_update(
         self,
     ):