Adjust wording

Author: fredericDelaporte

src/NHibernate/AdoNet/ConnectionManager.cs

@@ -407,7 +407,7 @@ private void EnsureTransactionIsCreated()
 		}
 
 		// Since v5.3
-		[Obsolete("Use CurrentTransaction instead, and check for nulls.")]
+		[Obsolete("Use CurrentTransaction instead, and check for null.")]
 		public ITransaction Transaction
 		{
 			get

src/NHibernate/ISession.cs

@@ -771,7 +771,7 @@ public partial interface ISession : IDisposable
 		/// Get the current Unit of Work and return the associated <c>ITransaction</c> object.
 		/// </summary>
 		// Since v5.3
-		[Obsolete("Use GetCurrentTransaction extension method instead, and check for nulls.")]
+		[Obsolete("Use GetCurrentTransaction extension method instead, and check for null.")]
 		ITransaction Transaction { get; }
 
 		/// <summary>

src/NHibernate/IStatelessSession.cs

@@ -55,7 +55,7 @@ public partial interface IStatelessSession : IDisposable
 
 		/// <summary>Get the current NHibernate transaction.</summary>
 		// Since v5.3
-		[Obsolete("Use CurrentTransaction instead, and check for nulls.")]
+		[Obsolete("Use CurrentTransaction instead, and check for null.")]
 		ITransaction Transaction { get; }
 
 		/// <summary>

src/NHibernate/Impl/AbstractSessionImpl.cs

@@ -38,7 +38,7 @@ public abstract partial class AbstractSessionImpl : ISessionImplementor
 
 		/// <summary>Get the current NHibernate transaction.</summary>
 		// Since v5.3
-		[Obsolete("Use CurrentTransaction instead, and check for nulls.")]
+		[Obsolete("Use CurrentTransaction instead, and check for null.")]
 		public ITransaction Transaction => ConnectionManager.Transaction;
 
 		/// <summary>