fix IsNew failing test:

- resolves todo re inconsistent exception types
- increases test case coverage for possible parameter combinations

there are numerous other methods that need their exception handling changed similarly, in a new PR

PetaPoco.Tests.Unit/DatabaseTests.cs

@@ -157,7 +157,12 @@ public void IsNew_GivenInvalidArguments_ShouldThrow()
         {
             Should.Throw<ArgumentNullException>(() => DB.IsNew(null));
             Should.Throw<ArgumentNullException>(() => DB.IsNew(null, null));
+            Should.Throw<ArgumentNullException>(() => DB.IsNew(null, default));
+            Should.Throw<ArgumentNullException>(() => DB.IsNew(null, null));
+            Should.Throw<ArgumentNullException>(() => DB.IsNew("PrimaryKey", null));
 
+            Should.Throw<ArgumentException>(() => DB.IsNew(string.Empty, null));
+            Should.Throw<ArgumentException>(() => DB.IsNew(string.Empty, default));
             Should.Throw<ArgumentException>(() => DB.IsNew("MissingId", new { }));
 
             Should.Throw<InvalidOperationException>(() => DB.IsNew(new TransactionLog()));

PetaPoco/Database.cs

@@ -2733,11 +2733,12 @@ public bool IsNew(object poco)
         }
 
         /// <inheritdoc/>
-        /// <exception cref="ArgumentException"><paramref name="primaryKeyName"/> is null or empty.</exception>
-        /// <exception cref="ArgumentNullException"><paramref name="poco"/> is null.</exception>
+        /// <exception cref="ArgumentException"><paramref name="primaryKeyName"/> is empty.</exception>
+        /// <exception cref="ArgumentNullException"><paramref name="poco"/> or <paramref name="primaryKeyName"/> is null.</exception>
         public bool IsNew(string primaryKeyName, object poco)
         {
-            // TODO: Inconsistent use of `ArgumentNullException` vs `ArgumentException` for null/empty string params.
+            if (primaryKeyName == null)
+                throw new ArgumentNullException(nameof(primaryKeyName));
             if (string.IsNullOrEmpty(primaryKeyName))
                 throw new ArgumentException(nameof(primaryKeyName));
             if (poco == null)