Support null as an empty source

[abelsromero]

convert and load fail with a java assert exception when input is null String.
For satefy we should consider null as an empty value that returns an empty document, for both convert and load.

In the case of convert with Reader I'd keep the current exception, at most, add Objects.requireNonNull(reader, "reader"); to

asciidoctorj/asciidoctorj-core/src/main/java/org/asciidoctor/jruby/internal/IOUtils.java

Line 22 in b190250

try (Scanner scanner = new Scanner(reader).useDelimiter("\\A")){

so the users gets a hint of where the issue is without adding too much complexity.

java.lang.NullPointerException: reader
	at java.base/java.util.Objects.requireNonNull(Objects.java:233)
	at org.asciidoctor.jruby.internal.IOUtils.readFull(IOUtils.java:24)
	at org.asciidoctor.jruby.internal.JRubyAsciidoctor.convert(JRubyAsciidoctor.java:351)
	at org.asciidoctor.jruby.internal.JRubyAsciidoctor.convert(JRubyAsciidoctor.java:358)
	at org.asciidoctor.jruby.ast.impl.SectionImplTest.temporal(SectionImplTest.java:28)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
	at