ZodString.minLength defaults to null to be consistent with maxLength

[Balastrong]

This PR fixes #1246.

minLength and maxLength are inconsistent in ZodString, this PR makes the behaviour more consistent by returning null if no max is defined (not changed) and if no min is defined (instead of -Infinity).

This PR also improves the syntax by removing the incorrect usage of Array.map that had side effects & no return).

A couple tests are also added to make sure it works as intended.

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[ghost]

Looking at other types in the library, it seems to me that the most consistent API would be to return number | null for both min and max, rather than returning number. Some examples below:

// ZodNumber
    get minValue(): number | null;
    get maxValue(): number | null;

// ZodArrayDef
    minLength: {
        value: number;
        message?: string;
    } | null;
    maxLength: {
        value: number;
        message?: string;
    } | null;

// ZodSetDef
    minSize: {
        value: number;
        message?: string;
    } | null;
    maxSize: {
        value: number;
        message?: string;
    } | null;

Therefore, I would suggest this instead:

// ZodString
    get minLength(): number | null;
    get maxLength(): number | null;

[Balastrong]

Hey @ealmansi-uc, I guess you're right. I have updated the PR to reflect the same logic used in ZodNumber and use number | null. Should look better now. Thank you!

[ghost]

@Balastrong LGTM now, although I'm not a contributor to this repo 😅 you'll probably want to update the PR's title
keep in mind this is a breaking change, I'm not sure how conservative this project is around breaking changes

[colinhacks]

Thanks, good PR!