Fix nullable Guid ToString is not translated correctly on some dialects (#2046)

Author: hazzik

src/NHibernate.Test/Async/NHSpecificTest/NH3426/Fixture.cs

@@ -97,5 +97,18 @@ public async Task CompareStringColumnWithGuidToStringAsync()
 				Assert.That(list, Has.Count.EqualTo(0));
 			}
 		}
+
+		[Test]
+		public async Task CompareStringColumnWithNullableGuidToStringAsync()
+		{
+			using (var session = OpenSession())
+			{
+				var list = await (session.Query<Entity>()
+				                  .Where(x => ((Guid?) x.Id).ToString() == x.Id.ToString())
+				                  .ToListAsync());
+
+				Assert.That(list, Has.Count.EqualTo(1));
+			}
+		}
 	}
 }

src/NHibernate.Test/NHSpecificTest/NH3426/Fixture.cs

@@ -85,5 +85,18 @@ public void CompareStringColumnWithGuidToString()
 				Assert.That(list, Has.Count.EqualTo(0));
 			}
 		}
+
+		[Test]
+		public void CompareStringColumnWithNullableGuidToString()
+		{
+			using (var session = OpenSession())
+			{
+				var list = session.Query<Entity>()
+				                  .Where(x => ((Guid?) x.Id).ToString() == x.Id.ToString())
+				                  .ToList();
+
+				Assert.That(list, Has.Count.EqualTo(1));
+			}
+		}
 	}
 }

src/NHibernate/Linq/Functions/StringGenerator.cs

@@ -317,21 +317,15 @@ public IHqlGeneratorForMethod GetMethodGenerator(MethodInfo method)
 
 	public class ToStringHqlGeneratorForMethod : IHqlGeneratorForMethod
 	{
-		private static readonly System.Type _guidType = typeof(Guid);
-		
-		public IEnumerable<MethodInfo> SupportedMethods
-		{
-			get { throw new NotSupportedException(); }
-		}
+		public IEnumerable<MethodInfo> SupportedMethods => throw new NotSupportedException();
 
 		public HqlTreeNode BuildHql(MethodInfo method, Expression targetObject, ReadOnlyCollection<Expression> arguments, HqlTreeBuilder treeBuilder, IHqlExpressionVisitor visitor)
 		{
-			if (targetObject.Type == _guidType)
-			{
-				return treeBuilder.MethodCall("strguid", visitor.Visit(targetObject).AsExpression());
-			}
+			var methodName = targetObject.Type == typeof(Guid) || targetObject.Type == typeof(Guid?)
+				? "strguid"
+				: "str";
 
-			return treeBuilder.MethodCall("str", visitor.Visit(targetObject).AsExpression());
+			return treeBuilder.MethodCall(methodName, visitor.Visit(targetObject).AsExpression());
 		}
 	}
 }