Allow empty string as version for find_compatible_in_house (#2430)

The empty string is used in several locations to indicate the newest
version. Interpret it as such instead of rejecting it as invalid.

docs/changelog/2429.bugfix.rst

@@ -0,0 +1 @@
+Fix fallback handling of downloading wheels for bundled packages - by :user:`schaap`.

src/virtualenv/seed/wheels/acquire.py

@@ -83,7 +83,7 @@ def _find_downloaded_wheel(distribution, version_spec, for_py_version, to_folder
 def find_compatible_in_house(distribution, version_spec, for_py_version, in_folder):
     wheels = discover_wheels(in_folder, distribution, None, for_py_version)
     start, end = 0, len(wheels)
-    if version_spec is not None:
+    if version_spec is not None and version_spec != "":
         if version_spec.startswith("<"):
             from_pos, op = 1, lt
         elif version_spec.startswith("=="):

tests/unit/seed/wheels/test_acquire_find_wheel.py

@@ -4,12 +4,18 @@
 from virtualenv.seed.wheels.embed import BUNDLE_FOLDER, MAX, get_embed_wheel
 
 
-def test_find_latest(for_py_version):
+def test_find_latest_none(for_py_version):
     result = find_compatible_in_house("setuptools", None, for_py_version, BUNDLE_FOLDER)
     expected = get_embed_wheel("setuptools", for_py_version)
     assert result.path == expected.path
 
 
+def test_find_latest_string(for_py_version):
+    result = find_compatible_in_house("setuptools", "", for_py_version, BUNDLE_FOLDER)
+    expected = get_embed_wheel("setuptools", for_py_version)
+    assert result.path == expected.path
+
+
 def test_find_exact(for_py_version):
     expected = get_embed_wheel("setuptools", for_py_version)
     result = find_compatible_in_house("setuptools", f"=={expected.version}", for_py_version, BUNDLE_FOLDER)