ListingContentConverters throws a NullPointerException when converting a block with null content

[wilkinsona]

Caused by: java.lang.NullPointerException	
	at org.jruby.RubyEncoding.doEncodeUTF8(org/jruby/RubyEncoding.java:222)	
	at org.jruby.RubyString.encodeBytelist(org/jruby/RubyString.java:6361)	
	at org.jruby.RubyString.<init>(org/jruby/RubyString.java:386)	
	at org.jruby.RubyString.newString(org/jruby/RubyString.java:477)	
	at org.jruby.Ruby.newString(org/jruby/Ruby.java:3425)	
	at io.spring.asciidoctor.backend.codetools.ListingContentConverters.content(io/spring/asciidoctor/backend/codetools ListingContentConverters.java:56)

listingBlock.getContent() was returning null due to a bug in the Spring Asciidoctor Extensions. I'm not sure if it's possible to trigger the problem in a completely hand-written document so we may not see the problem again. Perhaps it's worth handling null content more gracefully anyway?