fix: SafeInfo guard can be null

[schmanu]

What this PR changes

Changes the SafeInfo's guard field to be nullable.

Why?

This line converts the 0 address which is in the safe-contract to null:
https://github.com/safe-global/safe-client-gateway/blob/main/src/routes/safes/converters.rs#L53

Also here is an example Safe response without an transaction guard defined:

{
    "address": {
        "value": "0x180E0F9729EB4DD2d740052CB314dC918Ac0570B"
    },
    "chainId": "4",
    "nonce": 5,
    "threshold": 1,
    "owners": [
        {
            "value": "0x3819b800c67Be64029C1393c8b2e0d0d627dADE2"
        }
    ],
    "implementation": {
        "value": "0xd9Db270c1B5E3Bd161E8c8503c55cEABeE709552"
    },
    "modules": [
        {
            "value": "0xCFbFaC74C26F8647cBDb8c5caf80BB5b32E43134"
        },
        {
            "value": "0xaDa0e6Fe72A212240A4b5e7A867c6686e8aF0956"
        },
        {
            "value": "0x4753E44EE22b5a4c9D16877e104D761bB83937A1"
        }
    ],
    "fallbackHandler": {
        "value": "0xf48f2B2d2a534e402487b3ee7C18c33Aec0Fe5e4"
    },
    "guard": null,
    "version": "1.3.0",
    "implementationVersionState": "UP_TO_DATE",
    "collectiblesTag": "1657556137",
    "txQueuedTag": "1657556137",
    "txHistoryTag": "1657552495"
}

[github-actions]

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Type	Occurrences	Fixable
Errors	0	0
Warnings	0	0
Ignored	0	N/A

• Result: ✅ success
• Annotations: 0 total

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