DynamicTablesPkg: Clear pointer in node creation fcts

DynamicTablesPkg: Clear pointer in node creation fcts

The following functions:
- AmlCreateRootNode()
- AmlCreateObjectNode()
- AmlCreateDataNode()
create a node and return it by populating a pointer. This pointer
should only be considered/used if the function returns successfully.
Otherwise, the value stored in this pointer should be ignored.

For their error handling, some other functions assume that this
pointer is reset to NULL if an error occurs during a node creation.
To make this assumption correct, explicitly clear this input pointer.

Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>
Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>