Fix the empty price_s case and self.instruments in SBBStrategyEMA.

[ChiahungTai]

Description

1. Deal with price_s.empty:
2. Deal with instruments is List

Motivation and Context

How Has This Been Tested?

• [x] Pass the test by running: pytest qlib/tests/test_all_pipeline.py under upper directory of qlib.
• [ ] If you are adding a new feature, test on your own test scripts.

Screenshots of Test Results (if appropriate):

1. Pipeline test:
2. Your own tests:

Types of changes

• [x] Fix bugs
• [ ] Add new feature
• [ ] Update documentation

[ChiahungTai]

@Fivele-Li @you-n-g, Looks like the pylint has some problems, I think that is not my code's problem. Can you guys give me some suggestions to fix it?