[BUG] when param in where not constant

[zamis]

Version
Which LiteDB version/OS/.NET framework version are you using. (REQUIRED)
5.0.5 on master

Describe the bug
A clear and concise description of what the bug is.

when
db.GetCollection("Person").Find(x => x.Sex == PersonSex.Female);

• is ok

when
var PersonSexFemale = PersonSex.Female;
var p = db.GetCollection("Person").Find(x => x.Sex == PersonSexFemale);

• Exception
  [xUnit.net 00:00:44.75] LiteDB.Tests.Database.Enum_Tests.FindFemale [FAIL]
  [xUnit.net 00:00:44.76] System.NullReferenceException : Object reference not set to an instance of an object.
  [xUnit.net 00:00:44.76] Stack Trace:
  [xUnit.net 00:00:44.76] E:\Projects\TeamMe\LiteDB\LiteDB\Client\Mapper\Linq\LinqExpressionVisitor.cs(432,0): at LiteDB.LinqExpressionVisitor.VisitBinary(BinaryExpression node)
  [xUnit.net 00:00:44.76] at System.Linq.Expressions.BinaryExpression.Accept(ExpressionVisitor visitor)
  [xUnit.net 00:00:44.76] at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
  [xUnit.net 00:00:44.76] at System.Linq.Expressions.ExpressionVisitor.VisitLambda[T](Expression1 node) [xUnit.net 00:00:44.76] E:\Projects\TeamMe\LiteDB\LiteDB\Client\Mapper\Linq\LinqExpressionVisitor.cs(95,0): at LiteDB.LinqExpressionVisitor.VisitLambda[T](Expression1 node)
  [xUnit.net 00:00:44.76] at System.Linq.Expressions.Expression1.Accept(ExpressionVisitor visitor) [xUnit.net 00:00:44.76] at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node) [xUnit.net 00:00:44.76] E:\Projects\TeamMe\LiteDB\LiteDB\Client\Mapper\Linq\LinqExpressionVisitor.cs(64,0): at LiteDB.LinqExpressionVisitor.Resolve(Boolean predicate) [xUnit.net 00:00:44.76] E:\Projects\TeamMe\LiteDB\LiteDB\Client\Mapper\BsonMapper.cs(175,0): at LiteDB.BsonMapper.GetExpression[T,K](Expression1 predicate)
  [xUnit.net 00:00:44.76] E:\Projects\TeamMe\LiteDB\LiteDB\Client\Database\Collections\Find.cs(54,0): at LiteDB.LiteCollection1.Find(Expression1 predicate, Int32 skip, Int32 limit)
  [xUnit.net 00:00:44.76] e:\Projects\TeamMe\LiteDB\LiteDB.Tests\Database\Enum_Tests.cs(42,0): at LiteDB.Tests.Database.Enum_Tests.FindFemale()

Code to Reproduce
Write a small snippet to isolate your bug and could be possible to our team test. (REQUIRED)

public class Enum_Tests
    {
        public enum PersonSex
        {
            Male,
            Female
        }

        public class Person
        {
            public int Id { get; set; }
            public string Fullname { get; set; }
            public PersonSex Sex { get; set; }
        }


        [Fact]
        public void FindFemale()
        {
            using (var f = new TempFile())
            {
                using (var db = new LiteDatabase(f.Filename))
                {
                    var col = db.GetCollection<Person>("Person");

                    col.Insert(new Person { Fullname = "John", Sex = PersonSex.Male });
                    col.Insert(new Person { Fullname = "Doe", Sex = PersonSex.Male });
                    col.Insert(new Person { Fullname = "Joana", Sex = PersonSex.Female });
                    col.Insert(new Person { Fullname = "Marcus", Sex = PersonSex.Male });
                }
                // close datafile

                using (var db = new LiteDatabase(f.Filename))
                {
                    var PersonSexFemale = PersonSex.Female;
                    var p = db.GetCollection<Person>("Person").Find(x => x.Sex == PersonSexFemale);

                    p.Count().Should().Be(1);
                }
            }
        }
    }

Expected behavior
A clear and concise description of what you expected to happen.

Screenshots/Stacktrace
If applicable, add screenshots/stacktrace

Additional context
Add any other context about the problem here.

[lbnascimento]

@zamis This issue was fixed in the master and its fix will be present in the next incremental release.