Fix RuntimeError when trying to collect package with "__init__.py" only

Fixes https://github.com/pytest-dev/pytest/issues/4344.

src/_pytest/main.py

@@ -584,7 +584,13 @@ class Session(nodes.FSCollector):
             # Module itself, so just use that. If this special case isn't taken, then all
             # the files in the package will be yielded.
             if argpath.basename == "__init__.py":
-                yield next(m[0].collect())
+                try:
+                    yield next(m[0].collect())
+                except StopIteration:
+                    # The package collects nothing with only an __init__.py
+                    # file in it, which gets ignored by the default
+                    # "python_files" option.
+                    pass
                 return
             yield from m