x-pack/filebeat/input/awss3: fix priority of region name configurations

The code currently prioritises the region_name configuration, even when
it is not provided, against the claims of the documentation. Make a
check whether it is empty before claiming a conflict and using it.

CHANGELOG.next.asciidoc

@@ -147,6 +147,7 @@ https://github.com/elastic/beats/compare/v8.8.1\...main[Check the HEAD diff]
 - Improve error reporting and fix IPv6 handling of TCP and UDP metric collection. {pull}35996[35996]
 - Fix handling of NUL-terminated log lines in Fortinet Firewall module. {issue}36026[36026] {pull}36027[36027]
 - Make redact field configuration recommended in CEL input and log warning if missing. {pull}36008[36008]
+- Fix handling of region name configuration in awss3 input {pull}36034[36034]
 
 *Heartbeat*
 

x-pack/filebeat/input/awss3/input.go

@@ -119,7 +119,7 @@ func (in *s3Input) Run(inputContext v2.Context, pipeline beat.Pipeline) error {
 		if err != nil && in.config.RegionName == "" {
 			return fmt.Errorf("failed to get AWS region from queue_url: %w", err)
 		}
-		if regionName != in.config.RegionName {
+		if in.config.RegionName != "" && regionName != in.config.RegionName {
 			inputContext.Logger.Warnf("configured region disagrees with queue_url region: %q != %q: using %[1]q",
 				in.config.RegionName, regionName)
 			regionName = in.config.RegionName