opt: fix error in case of casted NULL arguments to AddGeometryColumn

This commit fixes an error that occurred when AddGeometryColumn was
called with NULL arguments that were cast to the type specified by the
function signature. #50992 already fixed the case when AddGeometryColumn
was called with bare NULL arguments, since those were detected by TypeCheck.
TypeCheck does not detect NULL arguments if they are cast to the correct
type.

This commit fixes the error by adding an explicit check in the optbuilder
that each argument is not null before calling the SQLFn of the
AddGeometryColumn overload.

Informs #50296

Release note (bug fix): Fixed an internal error that occurred when
AddGeometryColumn was called with NULL arguments. This now results in
a no-op and returns NULL.

pkg/sql/opt/exec/execbuilder/testdata/sql_fn

@@ -248,3 +248,8 @@ query T
 SELECT addgeometrycolumn('a', 'b', 3, NULL, 2);
 ----
 NULL
+
+query T
+SELECT addgeometrycolumn('a', 'b', NULL::string, 'c', 9223372036854775807:::INT8, 'd', NULL::int)
+----
+NULL

pkg/sql/opt/optbuilder/sql_fn.go

@@ -62,6 +62,9 @@ func (b *Builder) buildSQLFn(
 			))
 		}
 		exprs[i] = memo.ExtractConstDatum(info.args[i])
+		if exprs[i] == tree.DNull && !info.def.Properties.NullableArgs {
+			return b.factory.ConstructNull(info.ResolvedType())
+		}
 	}
 
 	// Get the SQL statement and parse it.