Adding IsNull, IsNotNull, IsEmpty, IsNotEmpty filters.

[mehmetuken]

No description provided.

[enisn]

Great job ✨🚀, thanks

I've just wanted some formatting changes to keep consistency in the project

[enisn]

Suggested change

	return Expression.Equal( Expression.Property(expressionBody, targetProperty.Name), Expression.Constant(null));
	case OperatorType.IsNotNull:
	return Expression.NotEqual( Expression.Property(expressionBody, targetProperty.Name), Expression.Constant(null));
	return Expression.Equal(Expression.Property(expressionBody, targetProperty.Name), Expression.Constant(null));
	case OperatorType.IsNotNull:
	return Expression.NotEqual(Expression.Property(expressionBody, targetProperty.Name), Expression.Constant(null));

[enisn]

Suggested change

	public static OperatorComparisonAttribute LessThanOrEqual { get; }
	public static OperatorComparisonAttribute IsNull { get; }
	public static OperatorComparisonAttribute IsNotNull { get; }
	public static OperatorComparisonAttribute LessThanOrEqual { get; }
	public static OperatorComparisonAttribute IsNull { get; }
	public static OperatorComparisonAttribute IsNotNull { get; }

[enisn]

Also, some of the tests are failing currently

Can you please update the following test according to the new logic?

AutoFilterer/tests/AutoFilterer.Tests/Types/OperatorQueryTests.cs

Lines 36 to 56 in e47ce55

	IQueryable<Book> GetAndQuery(IQueryable<Book> query, BookFilter_OperatorFilter_TotalPage f)
	{
	if (f.TotalPage.Eq != null)
	query = query.Where(x => x.TotalPage == f.TotalPage.Eq);
	if (f.TotalPage.Not != null)
	query = query.Where(x => x.TotalPage != f.TotalPage.Not);
	if (f.TotalPage.Gt != null)
	query = query.Where(x => x.TotalPage > f.TotalPage.Gt);
	if (f.TotalPage.Gte != null)
	query = query.Where(x => x.TotalPage >= f.TotalPage.Gte);
	if (f.TotalPage.Lt!= null)
	query = query.Where(x => x.TotalPage < f.TotalPage.Lt);
	if (f.TotalPage.Lte!= null)
	query = query.Where(x => x.TotalPage <= f.TotalPage.Lt);
	return query;

[mehmetuken]

Also, some of the tests are failing currently

Not-Nullable types getting error. Adding the type is nullable check binary expressions.

[enisn]

I've just referenced the issue which is found by CodeFactor
#49

It's ok to merge now. I'll refactor it later. Using if chains is not a good solution, but it's ok for now.