Prevent accidental manual setting of project name with -s=

Before `-s=name` would lead to project name being manually
set to '' rather than using FCM to setup a project name.
This is a correct from the point of view of optparse, but
might seem surprising to users.

cylc/rose/stem.py

@@ -340,7 +340,7 @@ def _ascertain_project(self, item):
         if re.search(r'^\.', item):
             item = os.path.abspath(os.path.join(os.getcwd(), item))
 
-        if project is not None:
+        if project:
             print(f"[WARN] Forcing project for '{item}' to be '{project}'")
             return project, item, item, '', ''
 

tests/unit/test_rose_stem_units.py

@@ -20,6 +20,7 @@
 import pytest
 from pytest import param
 from types import SimpleNamespace
+from typing import Any, Tuple
 
 from cylc.rose.stem import (
     ProjectNotFoundException,
@@ -34,6 +35,9 @@
 from metomi.rose.fs_util import FileSystemUtil
 
 
+Fixture = Any
+
+
 class MockPopen:
     def __init__(self, mocked_return):
         self.mocked_return = mocked_return
@@ -291,3 +295,45 @@ def test__deduce_mirror():
     }
     project = 'someproject'
     StemRunner._deduce_mirror(source_dict, project)
+
+
+@pytest.mark.parametrize(
+    'item, expect, stdout',
+    (
+        (
+            # Normally formed project=url
+            'foo=bar',
+            ('foo', 'bar'),
+            "Forcing project for 'bar' to be 'foo'",
+        ),
+        (
+            # Malformed project=url
+            '=foo',
+            ('', 'foo'),
+            None,
+        ),
+    )
+)
+def test_ascertain_project_if_name_supplied(
+    get_StemRunner: Fixture,
+    capsys: Fixture,
+    item: str,
+    expect: Tuple[str],
+    stdout: str,
+) -> None:
+    """Method gives sensible results for different CLI input.
+
+    Written because `-s=foo` leads to "item" including a leading = sign.
+    This led to project name being set to '', and skipping the FCM
+    calling logic, which the user might expect.
+    """
+    stemrunner = get_StemRunner({})
+    if stdout:
+        results = stemrunner._ascertain_project(item)
+        assert results[:2] == expect
+        assert stdout in capsys.readouterr().out
+    else:
+        with pytest.raises(
+            ProjectNotFoundException, match='is not a working copy'
+        ):
+            stemrunner._ascertain_project(item)