Location NPE fix

[dgault]

Issue was raised on the mailing list: http://lists.openmicroscopy.org.uk/pipermail/ome-users/2017-July/006557.html

From discussions since then it was decided to adapt the same contract as used in https://docs.oracle.com/javase/8/docs/api/java/io/File.html#getParentFile-- and return null if the parent does not exist rather than throwing a NullPointerException

To test:

• Ensure builds and tests are all green

[sbesson]

Closing/reopening to include #13 in the Travis build

[sbesson]

With the last fix, the tests now fail with

testParentNull(loci.common.utests.LocationTest)  Time elapsed: 0.007 sec  <<< FAILURE!
java.lang.AssertionError: nullParentFile expected:<null> but was:<null>
	at loci.common.utests.LocationTest.testParentNull(LocationTest.java:182)

[sbesson]

With the last commits, the tests passes and the behavior of Location matches the contract of java.io.File for the constructors and getParentFile. All makes sense to me. Only nitpicky comment before releasing is that I would like to review the Javadoc to make sure java.io.File is correctly referenced. I will try to open a Javadoc-only PR before cutting ome-common:5.3.3