Consistent resource-bundle descriptor validation (#272)

[elharo]

Fixes #272

Problem

validate() split bundle descriptors with StringUtils.split(artifactDescriptor, ":"),
which collapses empty segments, while downloadBundles() used
artifactDescriptor.split(":"), which preserves them. So a malformed descriptor
such as group:artifact::type (missing version) passed validate() and then
failed later with a confusing resolution error, never showing the clear
validation message.

Fix

validate() now uses the same split(":") logic as downloadBundles() and
additionally rejects descriptors where groupId, artifactId or version is empty
(e.g. a missing segment). Malformed descriptors now fail fast with the existing,
clear "resource bundle configured must specify..." message.

The reactor lookup in downloadBundles() compares with a non-null receiver
(s[0].equals(p.getGroupId())), so it is already null-safe.

Test

New unit test testValidateRejectsDescriptorWithEmptySegment: configures a
org.example:dep::jar descriptor and asserts validate() throws the validation
error. Verified the test fails without the fix (validate() let the descriptor
through) and passes with it. Full mvn verify -Prun-its (unit + failsafe ITs,
spotless/checkstyle/RAT) passes.