Avoid full report when all files are skipped

[Himanshuagrawal4]

Fixes #1008

Summary

• avoid printing a full scan report when every target is skipped
• keep normal report output unchanged when at least one file is scanned
• add functional and formatter unit coverage for skipped-only output

Why

When Bandit was run against a missing file, it printed "No issues identified" along with zero scanned lines before listing the skipped file. That made it look like the file had been scanned successfully.

Validation

• python -m unittest tests.functional.test_runtime tests.unit.formatters.test_text tests.unit.formatters.test_screen
• python -m flake8 bandit\formatters\text.py bandit\formatters\screen.py tests\functional\test_runtime.py tests\unit\formatters\test_text.py tests\unit\formatters\test_screen.py
• git diff --check