Fix NPE in ContextFunctionCatalogAutoConfiguration.wrap(..)

[olegz]

In the event user function factory method returns null, the stack trace is kind of vague

Caused by: java.lang.NullPointerException: null
	at java.util.concurrent.ConcurrentHashMap.putVal(ConcurrentHashMap.java:1011) ~[na:1.8.0_121]
	at java.util.concurrent.ConcurrentHashMap.put(ConcurrentHashMap.java:1006) ~[na:1.8.0_121]
	at org.springframework.cloud.function.context.config.ContextFunctionCatalogAutoConfiguration$ContextFunctionRegistry.wrap(ContextFunctionCatalogAutoConfiguration.java:434) ~[classes/:na]
	at org.springframework.cloud.function.context.config.ContextFunctionCatalogAutoConfiguration$Context
. . .

simple assertion would do

Object target = registration.getTarget();
Assert.notNull(target, message);
. . .