Self closing target in alt-trans matchrate crashes the import

Problem

The import will crash if in the imported xlf file (or xlf based file) there is alt-trans element containing self closing target element.

The reason for the error is, the code trys to compare the source content of the segment with the source content of the alt-trans source, in that case since the alt-trans source is self closing tag with no content, the check goes out of index when trying to the the content.

Example of reduced trans-unit with invalid alt-trans element:

<trans-unit ... >
<source>“{1}” on page {2}</source>
<target>„{1}“ auf Seite {2}</target>
<alt-trans match-quality="0.0" origin="machine-trans">
<target/>
</alt-trans>
...