Missing optional properties may produce literal nulls in output

[koral--]

Coming from that issue: mikepenz/AboutLibraries#461 through another one square/okhttp#5683 it turns out that if POM_DESCRIPTION is not set (I haven't found that in okhttp 4.x sources) then string with content "null" is produced in POM instead of not adding anything at all. Okhttp uses this plugin: https://github.com/square/okhttp/blob/master/okhttp/build.gradle#L1

I haven't tried to reproduce this nor further investigate if issue is located here or maybe in underlying Gradle implementation since my original issue at AboutLibraries has gone. I filled this issue because maybe someone else will be interested in investigation.

[gabrielittner]

Thanks for the report. It's fixed now.