feat: mempool transaction's tx raw may be nil

[zmcNotafraid]

What problem does this PR solve?

Fetch from mempool 's transaction's raw may be nil.We need to handle this situation.

Check List

Test

• unit test

  Task

  • none

[coveralls]

Pull Request Test Coverage Report for Build ef4ed678ab1cad5a6597cc58b5debe1df6e2b7a5-PR-1434

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.008%) to 55.34%

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Totals
Change from base Build 7277db2a5595ef63cf8a6df554e83df4e88b56e0:	0.008%
Covered Lines:	3285
Relevant Lines:	5936

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💛 - Coveralls