Account for empty string regexp in lineinfile (#41451)

* Use context managers for interacting with files

* Account for empty string as regexp

Rather than explicitly testing for None, also test for an empty string which will evaluate to False. An empty string regexp matches every line, which ends up replacing the incorrect line.

* Store line parameter in a variable

* Add tests

test/integration/targets/lineinfile/tasks/main.yml

@@ -717,3 +717,37 @@
   assert:
     that:
       - result.stat.checksum == 'eca8d8ea089d4ea57a3b87d4091599ca8b60dfd2'
+
+###################################################################
+# Issue 29443
+
+- name: Deploy the test file for lineinfile
+  copy:
+    src: test.txt
+    dest: "{{ output_dir }}/test.txt"
+  register: result
+
+- name: Assert that the test file was deployed
+  assert:
+    that:
+      - result is changed
+      - result.checksum == '5feac65e442c91f557fc90069ce6efc4d346ab51'
+      - result.state == 'file'
+
+- name: Insert a line in the file using an empty string as a regular expression
+  lineinfile:
+    path: "{{ output_dir }}/test.txt"
+    regexp: ''
+    line: This is line 6
+  register: insert_empty_regexp
+
+- name: Stat the file
+  stat:
+    path: "{{ output_dir }}/test.txt"
+  register: result
+
+- name: Assert that the file contents match what is expected
+  assert:
+    that:
+      - insert_empty_regexp is changed
+      - result.stat.checksum == '3baeade8eb2ecf4b01d70d541e9b8258b67c7f9f'