Fix Exception#full_message when given cause with empty backtrace

The issue leads to the following error:

```
NameError: undefined local variable or method `exception' for module Truffle::ExceptionOperations
<internal:core> core/truffle/exception_operations.rb:319:in `append_causes'
<internal:core> core/truffle/exception_operations.rb:270:in `full_message'
<internal:core> core/exception.rb:81:in `full_message'
```

Author: andrykonchin

core/exception/full_message_spec.rb

@@ -211,4 +211,16 @@ class << e
     e.full_message(highlight: false).lines.first.should =~ /RuntimeError/
     e.full_message(highlight: true).lines.first.should =~ /#{Regexp.escape("\e[1;4mRuntimeError\e[m")}/
   end
+
+  it "allows cause with empty backtrace" do
+    begin
+      raise RuntimeError.new("Some runtime error"), cause: RuntimeError.new("Some other runtime error")
+    rescue => e
+    end
+
+    full_message = e.full_message
+    full_message.should include "RuntimeError"
+    full_message.should include "Some runtime error"
+    full_message.should include "Some other runtime error"
+  end
 end