Emulate set.rb when hashing uninitialized Set

set.rb is implemented in pure Ruby, so its @hash instance var will
start out as nil. If you attempt to #hash an uninitialized Set,
the resulting hashcode is that of nil, rather than raising an
exception as seen in jruby#8393.

This fix emulates the nil hash. No other uses of the uninitialized
Set are fixed here, so it's still largely unusable until it has
been initialized.

See also jruby#8352.

Fixes jruby#8393.

core/src/main/java/org/jruby/ext/set/RubySet.java

@@ -924,6 +924,13 @@ public boolean eql(IRubyObject other) {
     @Override
     @JRubyMethod
     public RubyFixnum hash() { // @hash.hash
+        RubyHash hash = this.hash;
+
+        if (hash == null) {
+            // Emulate set.rb for jruby/jruby#8393
+            return ((RubyBasicObject) getRuntime().getNil()).hash();
+        }
+
         return hash.hash();
     }