Fix missing import streamlit as st in subsection files for empty folders

[Copilot]

When a subsection folder contains no recognized files, _generate_subsection produced a Python file missing import streamlit as st, causing runtime failures — the subsection header and footer always emit st.markdown() calls regardless of component count.

Changes

• streamlit_reportview.py — After _combine_components, unconditionally inject the three base imports (import streamlit as st, from pathlib import Path, section_dir = ...) into subsection_imports if not already present. Previously these only arrived via _generate_component_imports, which is never called when the component list is empty.

• tests/test_empty_folder.py — Three new tests: empty subsection contains all base imports; non-empty subsection is unaffected.

# Before fix — empty subsection generated file missing streamlit:
# (no imports)
# st.markdown(...)  ← NameError: name 'st' is not defined

# After fix — base imports always present:
import streamlit as st
from pathlib import Path
section_dir = Path(__file__).resolve().parent.parent

st.markdown(...)  # works correctly