Avoid putting resources with missing or empty sources into the xliff files

[ehoogerbeets]

• empty sources means nothing to localize, which is pointless and confusing for the translators
• for strings, do not put resources that have no source or a zero-length source into the extracted or new xliff files
• for arrays, allow elements of the arrays to have empty or zero-length strings in them, but do not put those into the extracted or new xliff files
  • make sure localization and pseudo localization still work with arrays that have empty elements