Allow empty list of names in pip module (#38789)

pip 10 gives exit code 1 for empty argument lists (pip < 10 gave exit 0)
see also https://github.com/pypa/pip/pull/4210

To still allow playbooks to pass when giving empty lists, don't call
pip in that case, but show a warning.

test/integration/targets/pip/tasks/pip.yml

@@ -208,3 +208,14 @@
   assert:
     that:
       - "venv_chdir.changed"
+
+# ansible#38785
+- name: allow empty list of packages
+  pip:
+    name: []
+  register: pip_install_empty
+
+- name: ensure empty install is successful
+  assert:
+    that:
+      - not pip_install_empty.changed