Fix empty env string case

cibuildwheel/environment.py

@@ -28,6 +28,9 @@ def split_env_items(env_string):
     >>> split_env_items('PATH2="something with spaces"')
     ['PATH2="something with spaces"']
     '''
+    if not env_string:
+        return []
+
     command_node = bashlex.parsesingle(env_string)
     result = []
 

unit_test/environment_test.py

@@ -66,3 +66,19 @@ def test_empty_var():
 
     assert environment_dict == {'CFLAGS': ''}
     assert environment_cmds == ['export CFLAGS=']
+
+def test_no_vars():
+    environment_recipe = parse_environment('')
+
+    environment_dict = environment_recipe.as_dictionary(prev_environment={})
+    environment_cmds = environment_recipe.as_shell_commands()
+
+    assert environment_dict == {}
+    assert environment_cmds == []
+
+def test_no_vars_pass_through():
+    environment_recipe = parse_environment('')
+
+    environment_dict = environment_recipe.as_dictionary(prev_environment={'CIBUILDWHEEL': 'awesome'})
+
+    assert environment_dict == {'CIBUILDWHEEL': 'awesome'}