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>()

modules/nf-commons/src/test/nextflow/plugin/HttpPluginRepositoryTest.groovy

@@ -160,26 +160,19 @@ class HttpPluginRepositoryTest extends Specification {
 
     // ------------------------------------------------------------------------
 
-    def 'handle prefetch error when metadata service sends back incorrectly formatted response'() {
+    def 'handle prefetch when registry returns empty plugins list'() {
         given:
         wiremock.stubFor(get(urlEqualTo("/v1/plugins/dependencies?plugins=nf-fake&nextflowVersion=${BuildInfo.version}"))
             .willReturn(aResponse()
                 .withStatus(200)
-                .withBody("""{
-                  "not-plugins": [
-                    {
-                      "id": "nf-fake"
-                    }
-                  ]
-                }
-                """)))
+                .withBody('{"plugins": []}')))
 
         when:
         unit.prefetch([new PluginRef("nf-fake")])
 
         then:
-        def err = thrown PluginRuntimeException
-        err.message.startsWith("Unexpected error while fetching plugin metadata from: http://localhost")
+        noExceptionThrown()
+        unit.getPlugins() == [:]
     }
 
     // ------------------------------------------------------------------------