fix panic when marshaling interface field containing typed nil value

[C-Pro]

When attempting to marshal a field containing a nil value of a type implementing one of the Marshaller interfaces, generated code panics when attempting to call corresponding Marshal function on non-nil interface containing nil value.

Given these type definitions:

// easyjson:json
type WrapperType struct {
	Inner any `json:"Inner"`
}

// easyjson:json
type InterfaceType struct {
	Field1 int `json:"Field1"`
}

This code will cause a panic:

	var inner *InterfaceType
	wrapper := WrapperType{
		Inner: inner,
	}

	data, err := wrapper.MarshalJSON() // This will panic

In this PR I add extra check for nil interface value in the generated code and a test that fails before the fix and passes after.