Fix error trying to generate scripts warning when PATH not set

fixes #5558

news/5558.bugfix

@@ -0,0 +1 @@
+Fix error trying to generate scripts warning when PATH not set

src/pip/_internal/wheel.py

@@ -163,7 +163,7 @@ def message_about_scripts_not_on_PATH(scripts):
     # We don't want to warn for directories that are on PATH.
     not_warn_dirs = [
         os.path.normcase(i).rstrip(os.sep) for i in
-        os.environ["PATH"].split(os.pathsep)
+        os.environ.get("PATH", "").split(os.pathsep)
     ]
     # If an executable sits with sys.executable, we don't warn for it.
     #     This covers the case of venv invocations without activating the venv.

tests/unit/test_wheel.py

@@ -470,3 +470,16 @@ class TestMessageAboutScriptsNotOnPATH(object):
             scripts=[os.path.join('a', 'b', 'c')]
         )
         assert retval is None
+
+    def test_missing_PATH_env_treated_as_empty_PATH_env(self):
+        scripts = ['a/b/foo']
+
+        env = os.environ.copy()
+        del env['PATH']
+        with patch.dict('os.environ', env, clear=True):
+            retval_missing = wheel.message_about_scripts_not_on_PATH(scripts)
+
+        with patch.dict('os.environ', {'PATH': ''}):
+            retval_empty = wheel.message_about_scripts_not_on_PATH(scripts)
+
+        assert retval_missing == retval_empty