chore: ensure string var interacting with ethers.getContractAt

[chechu]

Description

Resolves #

Checklist

• I have updated the documentation to account for the changes in the code.
• If I added new functionality, I added tests covering it.
• If I fixed a bug, I added a test preventing this bug from silently reappearing again.
• My contribution follows Venus contribution guidelines.

[github-actions]

Package	Line Rate	Branch Rate	Health
contracts	100%	100%	✔
contracts.Admin	88%	41%	✔
contracts.Comptroller	100%	90%	✔
contracts.Comptroller.Diamond	95%	59%	✔
contracts.Comptroller.Diamond.facets	79%	66%	➖
contracts.Comptroller.Diamond.interfaces	100%	100%	✔
contracts.DelegateBorrowers	100%	89%	✔
contracts.Governance	20%	14%	❌
contracts.InterestRateModels	47%	67%	❌
contracts.Lens	44%	37%	❌
contracts.Liquidator	98%	83%	✔
contracts.Oracle	100%	100%	✔
contracts.PegStability	88%	84%	✔
contracts.Swap	92%	58%	✔
contracts.Swap.interfaces	100%	100%	✔
contracts.Swap.lib	81%	55%	✔
contracts.Tokens	100%	100%	✔
contracts.Tokens.Prime	96%	72%	✔
contracts.Tokens.Prime.Interfaces	100%	100%	✔
contracts.Tokens.Prime.libs	90%	76%	✔
contracts.Tokens.VAI	71%	46%	➖
contracts.Tokens.VRT	20%	9%	❌
contracts.Tokens.VTokens	61%	40%	➖
contracts.Tokens.XVS	19%	8%	❌
contracts.Utils	49%	29%	❌
contracts.VAIVault	50%	45%	➖
contracts.VRTVault	49%	36%	❌
contracts.XVSVault	64%	50%	➖
contracts.lib	75%	25%	➖
Summary	69% (3077 / 4477)	48% (1142 / 2356)	➖