libmsooxml: Don't process a blip in case Target="NULL" in relationships file.

ECMA-376 Office Open XML File Formats – Part 2
The Target attribute shall be a URI reference pointing to a
target resource.  The URI reference shall be a URI or a relative
reference.

The test file attached to the bug seems odd, so I did not add the
condition to all places where embedded objects are processed.

BUG:286700