Method execution should not error without value

- Do npe check when getting class type out from
  incoming value.
- Fix #572

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

@@ -183,8 +183,10 @@ public interface CommandExecution {
 			if (parameterName == null) {
 				return false;
 			}
+			Class<?> sourceType = paramValues.get(parameterName) != null ? paramValues.get(parameterName).getClass()
+					: null;
 			return paramValues.containsKey(parameterName) && conversionService
-					.canConvert(paramValues.get(parameterName).getClass(), parameter.getParameterType());
+					.canConvert(sourceType, parameter.getParameterType());
 		}
 
 		@Override

spring-shell-core/src/test/java/org/springframework/shell/command/CommandExecutionTests.java

@@ -100,6 +100,26 @@ public class CommandExecutionTests extends AbstractCommandTests {
 		assertThat(pojo1.method1Ctx).isNotNull();
 	}
 
+	@Test
+	public void testMethodArgWithoutValue() {
+		CommandRegistration r1 = CommandRegistration.builder()
+			.command("command1")
+			.description("help")
+			.withOption()
+				.longNames("arg1")
+				.description("some arg1")
+				.position(0)
+				.arity(OptionArity.EXACTLY_ONE)
+				.and()
+			.withTarget()
+				.method(pojo1, "method4")
+				.and()
+			.build();
+		execution.evaluate(r1, new String[]{"--arg1"});
+		assertThat(pojo1.method4Count).isEqualTo(1);
+		assertThat(pojo1.method4Arg1).isNull();
+	}
+
 	@Test
 	public void testMethodSinglePositionalArgs() {
 		CommandRegistration r1 = CommandRegistration.builder()