[Bug-gnuzilla] MOZ variables in confvars.sh should be assigned empty strings, not 0

[1gooddood1]

At least, so I believe.

Code snippet from old.configure:
>if test -n "$MOZ_TELEMETRY_REPORTING" || test -n "$MOZ_SERVICES_HEALTHREPORT" 
>|| test -n "$MOZ_CRASHREPORTER"; then
>  MOZ_DATA_REPORTING=1
>  ...
>fi

Icecat Mobile on F-Droid also assigns empty strings to the vars
(https://gitlab.com/Manizuca/IceCat/blob/master/makeicecat)
and I can personally confirm that using 'MOZ_SERVICES_HEALTHREPORT='
is what removes the healthreport checkbox in Fennec's settings entirely,
while setting it to 0 does not.

Though I'm not sure if this is what we want for every variable set by
Icecat.