Handle grep finds no matches.

Bazel is more strict about exit codes in genrules. In
vendor.mime.types.minimized, there are no no-empty lines, so the grep
fails. Update the command to succeed even though grep fails.

Test: m mimemap and verify contents of
    out/soong/.intermediates/frameworks/base/mime/vendor.mime.types.minimized/android_common/gen/vendor.mime.types
Change-Id: I292fa4cde584d2a620fd245469e34bbaa13df256