Maven plugin (spring-boot:run) produces NPE when empty argument is passed in plugin config

The Maven plugin needs to have much more robust/defensive code. It is way too easy to produce a NPE with ```mvn spring-boot:run``` by just doing this:
```xml
<plugin>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-maven-plugin</artifactId>
  <configuration>
    <arguments>
      <argument></argument>
    </arguments>
  </configuration>
</plugin>
```