fix empty link causing NullPointerException

[chonton]

When configuration has an empty link, a NullPointerException is thrown.

  <configuration>
    <links>
      <link></link>
    </links>
...
  </configuration>

[codecov-io]

Current coverage is 49.21% (diff: 100%)

Merging #693 into master will increase coverage by 0.03%

@@             master       #693   diff @@
==========================================
  Files           124        124          
  Lines          6357       6358     +1   
  Methods           0          0          
  Messages          0          0          
  Branches       1080       1080          
==========================================
+ Hits           3126       3129     +3   
+ Misses         2961       2960     -1   
+ Partials        270        269     -1   

Diff Coverage	File Path
•••••••••• 100%	.../main/java/io/fabric8/maven/docker/util/EnvUtil.java

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[rhuss]

Thanks ! LGTM