fix(sdk-coin-eth): guard BN construction in signFinal for EIP-1559 txns #3502

Job	Run time
Generate merge base API spec	9m 4s
Generate head API spec	8m 44s
Check specs identical	8s
API Spec Check	4s
Generate vacuum reports for API spec	-1s
Check breaking changes	-1s
Linter Override	0s
	17m 58s

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