fix: Prevent panic on validating nil pointer to slice field (#252)

[kaptinlin]

This pull request addresses the panic encountered when the gookit/validate library attempts to validate a struct that contains a nil pointer to a slice field, as reported in issue #252.

Problem:
When validating a struct with a field that is a nil pointer to a slice, the library would panic with an error message: reflect: call of reflect.Value.Len on zero Value. This is due to a call to Len() without first checking if the reflect.Value is valid and not nil.

Solution:
The proposed fix introduces a check that ensures the reflect.Value is valid and not nil before any call to Len(). This prevents the panic by safely handling nil pointers as if they point to empty slices, which aligns with the expected behavior of the validation library.

Example:
The issue can be reproduced and tested with the following example:

package main

import (
    "github.com/gookit/validate"
    "github.com/stretchr/testify/assert"
)

type Data struct {
    PtrStrings *[]string `validate:"strings"`
}

func main() {
    v := validate.New(&Data{})

    ok := v.Validate() // Before the fix, this would panic
    assert.True(t, ok) 
}

With the fix applied, the Validate method now correctly handles the nil pointer by treating it as an empty slice, and the assertion passes without a panic.

[coveralls]

Pull Request Test Coverage Report for Build 7382742445

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.03%) to 96.204%

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Totals
Change from base Build 7207994547:	-0.03%
Covered Lines:	2864
Relevant Lines:	2977

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💛 - Coveralls