An empty string literal passed to property substitution will generate an NPE.

The problem seems to lie in the changes that were made to the ch.qos.logback.core.subst.NodeToStringTransformer class. Tokenize returns a null Token in this case, which then fails at line 75 of the ch.qos.logback.core.subst.Parser class, where the switch tries to access the type filed on Token.

I have attached a small test case which you can unzip and run. The stack trace below should appear on stderr.
Unexpected error, server was not started: this is probably a bug.
java.lang.NullPointerException
at ch.qos.logback.core.subst.Parser.T(Parser.java:75)
at ch.qos.logback.core.subst.Parser.E(Parser.java:50)
at ch.qos.logback.core.subst.Parser.parse(Parser.java:46)
at ch.qos.logback.core.subst.NodeToStringTransformer.tokenizeAndParseString(NodeToStringTransformer.java:55)
at ch.qos.logback.core.subst.NodeToStringTransformer.handleVariable(NodeToStringTransformer.java:96)
at ch.qos.logback.core.subst.NodeToStringTransformer.compileNode(NodeToStringTransformer.java:72)
at ch.qos.logback.core.subst.NodeToStringTransformer.transform(NodeToStringTransformer.java:60)
at ch.qos.logback.core.subst.NodeToStringTransformer.substituteVariable(NodeToStringTransformer.java:48)
at ch.qos.logback.core.util.OptionHelper.substVars(OptionHelper.java:117)
at ch.qos.logback.core.joran.spi.InterpretationContext.subst(InterpretationContext.java:159)
at ch.qos.logback.core.joran.action.NestedBasicPropertyIA.body(NestedBasicPropertyIA.java:87)
at ch.qos.logback.core.joran.spi.Interpreter.callBodyAction(Interpreter.java:295)
at ch.qos.logback.core.joran.spi.Interpreter.characters(Interpreter.java:175)
at ch.qos.logback.core.joran.spi.EventPlayer.play(EventPlayer.java:57)
at ch.qos.logback.core.joran.GenericConfigurator.doConfigure(GenericConfigurator.java:149)
at ch.qos.logback.core.joran.GenericConfigurator.doConfigure(GenericConfigurator.java:135)
at ch.qos.logback.core.joran.GenericConfigurator.doConfigure(GenericConfigurator.java:99)
at ch.qos.logback.core.joran.GenericConfigurator.doConfigure(GenericConfigurator.java:76)
at ch.qos.logback.core.joran.GenericConfigurator.doConfigure(GenericConfigurator.java:68)
at LogbackTokenizerTest.main(LogbackTokenizerTest.java:17)