bundle-uri: stop sending invalid bundle configuration

bundle-uri: stop sending invalid bundle configuration

When bundle-URI info is requested by the client, the server responds
with all "bundle.*" config lines as key=value packet lines. On the
client-side, the received bundle config packet lines are always expected
to contain both a key and a value otherwise the client errors out during
parsing. The server performs no validation of the read bundle
configuration though which results in any misconfiguration on the
server-side, such as bundle configuration with an empty value, being
blindly sent to the client.

To avoid having the server transmit invalid configuration to clients,
only send bundle configuration that has non-empty values.

This change makes bundle-URI information sent by the server
syntactically correct, but semantically it still can be invalid. For
example the server may end up sending `bundle.bundle-1.creationToken`,
but be lacking a `bundle.bundle-1.uri` for that bundle. The `uri` is
mandatory, thus the client cannot process this bundle and will error
with the message:

    error: bundle 'bundle-1' has no uri

Fixing this would require a more complex solution, because bundles need
to be validated as a whole and not line-by-line. This is considered
outside the scope of this change.

Co-authored-by: Toon Claes <redacted>
Signed-off-by: Justin Tobler <redacted>
Signed-off-by: Toon Claes <redacted>
Signed-off-by: Junio C Hamano <redacted>