Systematic default values of Package Tracker variables

[koudis]

Package tracker variables shall have consistent defaults - OFF or ON or Empty string. Otherwise inconsisne t behaviur is observed and... that's not something that is wanted.

For example, the variable HTTP_HEADER:

BA_PACKAGE_VARS__HTTP_HEADER defaults to "NonExistentHTTPHEADER". When BA_PACKAGE_CMCONF_USE=OFF the IF(BA_PACKAGE_CMCONF_USE) block in STORAGE.cmake is skipped, so BA_PACKAGE_VARS_SET(HTTP_HEADER ...) is never called. The unconditional BA_PACKAGE_VARS_GET(HTTP_HEADER http_header) that follows then reads "NonExistentHTTPHEADER", which is truthy, and the code force-writes CMLIB_FILE_DOWNLOAD_HTTP_HEADER "NonExistentHTTPHEADER". Every CMLIB download in that configure run will send a malformed HTTP header. The existing test/ suite and any downstream project using BA_PACKAGE_CMCONF_USE=OFF for offline testing is affected.

Use an empty string as the default (consistent with ESCAPE_TEMPLATE_ARGS using OFF), or gate the BA_PACKAGE_VARS_GET/CMLIB_FILE_DOWNLOAD_HTTP_HEADER block inside IF(BA_PACKAGE_CMCONF_USE).