NH-3900 - Replace Assert.IsNullOrEmpty and Assert.IsNotNullOrEmpty to fluent syntax as it will be removed in NUnit 3.6.0

Author: hazzik

src/NHibernate.Test/Linq/DynamicQueryTests.cs

@@ -34,7 +34,7 @@ public void CanCahceDynamicLinq()
 			using (var log = new SqlLogSpy())
 			{
 				users.ToList();
-				Assert.IsNullOrEmpty(log.GetWholeLog());
+				Assert.That(log.GetWholeLog(), Is.Null.Or.Empty);
 			}
 		}
 	}

src/NHibernate.Test/NHSpecificTest/NH2420/Fixture.cs

@@ -31,16 +31,16 @@ private string FetchConnectionStringFromConfiguration()
 			string connectionString;
 			if (cfg.Properties.TryGetValue(Environment.ConnectionString, out connectionString))
 			{
-				Assert.IsNotNullOrEmpty(connectionString);
+				Assert.That(connectionString, Is.Not.Null.Or.Empty);
 				return connectionString;
 			}
 			string connectionStringName;
 			if (cfg.Properties.TryGetValue(Environment.ConnectionStringName, out connectionStringName))
 			{
 				var connectionStringSettings = ConfigurationManager.ConnectionStrings[connectionStringName];
-				Assert.IsNotNull(connectionStringSettings);
+				Assert.That(connectionStringSettings, Is.Not.Null);
 				connectionString = connectionStringSettings.ConnectionString;
-				Assert.IsNotNullOrEmpty(connectionString);
+				Assert.That(connectionString, Is.Not.Null.Or.Empty);
 				return connectionString;
 			}
 			else