Enhance exception handling for license execution when the URL tag of the license is empty.

[djjeon]

1. example: pom.xml

<license>
  <name/>
  <url/>
  <distribution/>
</license>

2. error message occurs during the execution of license:aggregate-download-licenses
   Execution default-cli of goal org.codehaus.mojo:license-maven-plugin:2.4.0:aggregate-download-licenses failed: Null URL

[slawekjaranowski]

@djjeon I miss that build not executed ... please reopen