failed to save xlf file

[MSakamaki]

problem

An exception will be issued and save will fail.
The xlf file generated by the latest ng x18n seems to have no <target/> element.

error xlf code (initial generate)

  <trans-unit id="i18nid" datatype="html">
    <source> source message </source>
    <context-group purpose="location">
      <context context-type="sourcefile">./path/to/soucefile.ts</context>
      <context context-type="linenumber">0</context>
    </context-group>
    <note priority="1" from="description">source message</note>
  </trans-unit>

cause

There seems to be an exception when saving the translation because there is no target element.

desired action

I will send you a pull request so that it can be saved.