Bridging to undefined clojure methods may throw NullPointerException

When bridging to an undefined clojure method, the `Var` lookup will return `null`. When this `null` is injected into the `PlasticField`, either an `AssertionError` will be thrown, or, if assertions are disabled, a `NullPointerException` will eventually be thrown from somewhere deep in the generated bytecode. A meaningful exception would be better in that case.