Feature: Enhance 1.4.1 contract checks for support proxy contract

[mmv08]

This PR:

• Closes https://github.com/safe-global/safe-contracts/issues/672 by implementing the same checks as in the 1.5.0 contract that validates that the address stored at the storage slot 0 is a contract (conventional storage proxy)

[github-actions[bot]]

Pull Request Test Coverage Report for Build 6610678646

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 95.2%

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Totals
Change from base Build 6589591326:	0.02%
Covered Lines:	368
Relevant Lines:	377

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💛 - Coveralls

[mmv08]

Code looks good to me, as for the rationale, is it just we don't want to accidentally run this migration on a proxy contract that isn't the standard Safe proxy contract (and overwrite some important storage that the proxy contract uses for other things)?

Yes and no. A correct proxy implementation should use something other than this storage slot because things may break. In this case, it's more about running a migration transaction without migrating anything. For version 1.5.0, though, it gets more critical because the guard interface changes there to add a module transaction check. The contract also implements migration methods for such cases (upgrade singleton together with a guard), so it was needed to avoid an issue where the singleton stays the same, but the guard updates. See https://github.com/safe-global/safe-contracts/pull/652#discussion_r1331387586