SPR-11581 Filter empty MIME types.

When Oracle JAI is installed, ImageIO.get(Reader|Writer)MIMETypes() both
return arrays with a leading empty string. It is unclear whether this is
a valid behavior from ImageIO (the Javadocs are not specific), but it is
not handled by BufferedImageHttpMessageConverter. That causes SpringMVC
to fail to initialize when JAI is installed.

- When looping over ImageIO getReaderMIMETypes() and getWriterMIMETypes()
  strip out any empty strings in BufferedImageHttpMessageConverter