GraphicsContext.setTextAlign(null) causes exceptions

If you set the text alignment attribute of a Canvas GraphicsContext to null then you get an exception, but the method leaves bad data in the buffer. The rendering side can recover from the bad data, but many intermediate operations may be lost by its conservative approach to recovering from internal exceptions. While it is potentially acceptable for the method to generate an NPE for a null input, it should not leave bad data in the buffer.