SQL geography::point null parameter error when parameter is not null?

I have struck similar issues with Geometry and Geography data types before. I think the issue is to do with where (and how) the optimizer decides to build the geometry in the plan.

I was having an issue with insert statement that was basically

INSERT INTO (ID,GEOM)
SELECT ID, Geometry::Point(ISNULL(X,0),ISNULL(Y,0),0)
FROM HeapTable
WHERE X is not NULL and Y is not NULL

After playing around and getting a little confused found that

INSERT INTO (ID,GEOM)
SELECT ID, Geometry::Point(ISNULL(X,0),ISNULL(Y,0),0)
FROM HeapTable;

INSERT INTO (ID,GEOM)
SELECT ID, Geometry::Point(ISNULL(X,0),ISNULL(Y,0),0)
FROM HeapTable
WHERE ISNULL(X,0) != 0;

both work but the following doesn't

INSERT INTO (ID,GEOM)
SELECT ID, Geometry::Point(X,Y,0)
FROM HeapTable
WHERE ISNULL(X,0) != 0;

For the first query the plan shows that the compute scalar is done before the filter

And the expression being computed has had the ISNULL function stripped from it.

<ColumnReference Column="Expr1005" />
<ScalarOperator ScalarString="[Geometry]::Point(CONVERT_IMPLICIT(float(53),[sandbox].[dbo].[TestNullGeomIssueSource].[X],0),CONVERT_IMPLICIT(float(53),[sandbox].[dbo].[TestNullGeomIssueSource].[Y],0),(0))">

I think the Point on the Geometry and Geography is flawed in not allowing NULLs to be input. Simply return a NULL as a result would be more correct in my opinion.

If you wish I can put up my entire test script for this.