Method execution should not error without value

[jvalkeal]

Essentially when execution target is method(via annotation model of manually registering method):

my-shell:>e2e anno required-value --arg1
Cannot invoke "Object.getClass()" because the return value of "java.util.Map.get(Object)" is null

NPE comes from

spring-shell/spring-shell-core/src/main/java/org/springframework/shell/command/CommandExecution.java

Line 187 in ee7a9da

.canConvert(paramValues.get(parameterName).getClass(), parameter.getParameterType());

.