[REGRESSION] MJAVADOC-791 causes forked Maven execution fail if any toolchains or settings file isn't present

When JavadocUtil invokes another Maven process (fork()) it passes global/user toolchains/settings file path from MavenSession. Those values are always populated – if not explciitly provided they will contain the default value, but there is no guarantee that the default value exist. We need to check the value whether it exists and only pass it on if it exists.