Add validation logic for non-nullable types.

mojo/public/interfaces/bindings/tests/data/validation/conformance_mthd8_unexpected_null_string.data

Index: mojo/public/interfaces/bindings/tests/data/validation/conformance_mthd8_unexpected_null_string.data
diff --git a/mojo/public/interfaces/bindings/tests/data/validation/conformance_mthd8_unexpected_null_string.data b/mojo/public/interfaces/bindings/tests/data/validation/conformance_mthd8_unexpected_null_string.data
new file mode 100644
index 0000000000000000000000000000000000000000..104dddb0bd6808b9f2de8fcc0011460f4fc8532e
--- /dev/null
+++ b/mojo/public/interfaces/bindings/tests/data/validation/conformance_mthd8_unexpected_null_string.data
@@ -0,0 +1,31 @@
+[dist4]message_header  // num_bytes
+[u4]2                  // num_fields
+[u4]8                  // name
+[u4]0                  // flags
+[anchr]message_header
+
+[dist4]method8_params  // num_bytes
+[u4]1                  // num_fields
+[dist8]param0_ptr      // param0
+[anchr]method8_params
+
+[anchr]param0_ptr
+[dist4]array_param       // num_bytes
+[u4]3                    // num_elements
+[u8]0                    // A null pointer, which is okay.
+[dist8]nested_array_ptr
+[u8]0                    // A null pointer, which is okay.
+[anchr]array_param
+
+[anchr]nested_array_ptr
+[dist4]nested_array  // num_bytes
+[u4]2                // num_elements
+[dist8]string_ptr
+[u8]0                // An unexpected null pointer.
+[anchr]nested_array
+
+[anchr]string_ptr
+[dist4]string  // num_bytes
+[u4]5          // num_elements
+0 1 2 3 4
+[anchr]string