Fix HttpPluginRepository empty plugin list check (#6650)

Revert condition from `!decoded.plugins` to `decoded.plugins == null`
to avoid throwing an exception when the registry returns an empty list.
In Groovy, an empty list is falsy, so the previous check incorrectly
treated a valid empty response as a parse failure.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Paolo Di Tommaso <paolo.ditommaso@gmail.com>

Author: pditommaso

modules/nf-commons/src/main/nextflow/plugin/HttpPluginRepository.groovy

@@ -168,7 +168,7 @@ class HttpPluginRepository implements PrefetchUpdateRepository {
         }
         try {
             final ListDependenciesResponse decoded = encoder.decode(body)
-            if( !decoded.plugins ) {
+            if( decoded.plugins == null ) {
                 throw new PluginRuntimeException("Failed to download plugin metadata: Failed to parse response body")
             }
             final result = new HashMap<String, PluginInfo>()