NPE on getRepoTags()

[outofcoffee]

This is similar to the issue fixed in cd15bec.

A NPE can be thrown when checking disk space when getRepoTags() is null. This seems to occur when an image is in the following format:

<registry name>/<repo>/<image>

...but hasn't been pushed to the registry yet (or at least the local image store does not have a record of it happening).

Stacktrace:

2016-08-10 23:07:29.864 [Test worker] WARN  o.testcontainers.DockerClientFactory - [] Encountered and ignored error while checking disk space
java.lang.NullPointerException: null
    at java.util.Objects.requireNonNull(Objects.java:203) ~[na:1.8.0_65]
    at java.util.Arrays$ArrayList.<init>(Arrays.java:3813) ~[na:1.8.0_65]
    at java.util.Arrays.asList(Arrays.java:3800) ~[na:1.8.0_65]
    at org.testcontainers.DockerClientFactory.lambda$checkDiskSpace$0(DockerClientFactory.java:146) [testcontainers-cd15becf1ef907ad41843554aef5924a9a1b1b3b.jar:na]
    at java.util.stream.MatchOps$1MatchSink.accept(MatchOps.java:90) ~[na:1.8.0_65]
    at java.util.ArrayList$ArrayListSpliterator.tryAdvance(ArrayList.java:1351) ~[na:1.8.0_65]
    at java.util.stream.ReferencePipeline.forEachWithCancel(ReferencePipeline.java:126) ~[na:1.8.0_65]
    at java.util.stream.AbstractPipeline.copyIntoWithCancel(AbstractPipeline.java:498) ~[na:1.8.0_65]
    at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:485) ~[na:1.8.0_65]
    at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471) ~[na:1.8.0_65]
    at java.util.stream.MatchOps$MatchOp.evaluateSequential(MatchOps.java:230) ~[na:1.8.0_65]
    at java.util.stream.MatchOps$MatchOp.evaluateSequential(MatchOps.java:196) ~[na:1.8.0_65]
    at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234) ~[na:1.8.0_65]
    at java.util.stream.ReferencePipeline.anyMatch(ReferencePipeline.java:449) ~[na:1.8.0_65]
    at org.testcontainers.DockerClientFactory.checkDiskSpace(DockerClientFactory.java:146) [testcontainers-cd15becf1ef907ad41843554aef5924a9a1b1b3b.jar:na]
    at org.testcontainers.DockerClientFactory.checkDiskSpaceAndHandleExceptions(DockerClientFactory.java:131) [testcontainers-cd15becf1ef907ad41843554aef5924a9a1b1b3b.jar:na]
    at org.testcontainers.DockerClientFactory.client(DockerClientFactory.java:103) [testcontainers-cd15becf1ef907ad41843554aef5924a9a1b1b3b.jar:na]
    at org.testcontainers.DockerClientFactory.client(DockerClientFactory.java:74) [testcontainers-cd15becf1ef907ad41843554aef5924a9a1b1b3b.jar:na]

Offending line

[kunickiaj]

I already submitted a pull request for this #201

[outofcoffee]

Fixed by #201