fix: ignore empty CIBW_ARCHS (#2541)

cibuildwheel/options.py

@@ -657,7 +657,9 @@ class Options:
         )
         requires_python = None if requires_python_str is None else SpecifierSet(requires_python_str)
 
-        archs_config_str = args.archs or self.reader.get("archs", option_format=ListFormat(sep=" "))
+        archs_config_str = args.archs or self.reader.get(
+            "archs", option_format=ListFormat(sep=" "), ignore_empty=True
+        )
         architectures = Architecture.parse_config(archs_config_str, platform=self.platform)
 
         # Process `--only`

unit_test/main_tests/main_platform_test.py

@@ -278,3 +278,12 @@ def test_pyodide_on_windows(monkeypatch, capsys):
 
     assert exit.value.code == 2
     assert "Building for pyodide is not supported on Windows" in err
+
+
+def test_empty_archs_platform(platform, intercepted_build_args, monkeypatch):
+    monkeypatch.setenv("CIBW_ARCHS", "")
+
+    main()
+
+    options = intercepted_build_args.args[0]
+    assert options.globals.architectures == Architecture.auto_archs(platform)