ImageMetadata: return length 0 for unknown axes

Returning a length of 1 for unknown axes is confusing and misleading.
Return 0 instead.

Clarified this default value in the ImageMetadata javadoc.

Added a test to MetadataTest confirming the metadata values for unknown
axes.
MetadataTest: add javadoc for testAddingAxes

Added javadoc and comments for this test.