Fixed nil pointer error

[Smnrgcl]

Hello,

The error encountered in the project is a runtime error in the form of "invalid memory address or nil pointer dereference". Upon investigating the source of this error, I noticed a nil pointer error in the github.com/octoposprime/op-be-graphql/pkg/presentation/dto/model.(*UserDto).ToModel function.

As a solution, necessary nil pointer checks were added in the (*UserDto).ToModel function, thus resolving the error. The changes were made in the relevant lines of the user.go file.

With these changes added, the function in question is now prevented from unexpectedly working with a nil object.

As seen, the solution to the error is quite simple, but I wanted to share this solution for the benefit of other developers who may encounter similar situations.

I have also included code snippets for detailed review.

// Modified Code Sections

func (u *UserDto) ToModel() *User {
    u.ModelData.UserBase = new(UserBase) // Initialize the UserBase field
    // Other code...
}

[Sddilora]

Can you please add a description to the title of the pr

[Sddilora]

Closes #6