Problem: nil pointer error with legacy tx format

[yihuang]

Closes: #XXX

Description

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For contributor use:

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

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For admin use:

• Added appropriate labels to PR (ex. WIP, R4R, docs, etc)
• Reviewers assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.

Project coverage is 46.18%. Comparing base (4fb45a3) to head (6e29acd).
Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
x/evm/types/eth.go	0.00%	5 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #562      +/-   ##
===========================================
- Coverage    46.20%   46.18%   -0.02%     
===========================================
  Files          181      181              
  Lines        18146    18151       +5     
===========================================
- Hits          8384     8383       -1     
- Misses        9043     9048       +5     
- Partials       719      720       +1     

Files with missing lines	Coverage Δ
x/evm/types/eth.go	43.66% <0.00%> (-4.83%)	⬇️