fix: omit empty content from an LlmResponse built from a content-less candidate

A candidate with no content parts (for example one blocked for safety) now
yields an LlmResponse with null content instead of an empty Content, unless the
turn finished normally.

PiperOrigin-RevId: 967732960

Author: Mateusz Krawiec

core/src/commonMain/kotlin/com/google/adk/kt/models/LlmResponse.kt

@@ -167,7 +167,11 @@ data class LlmResponse(
         candidate?.finishReason ?: response.promptFeedback?.blockReason?.toFinishReason()
 
       return LlmResponse(
-        content = candidate?.content,
+        // Keep content only when it has parts or the turn finished normally, matching Python ADK.
+        content =
+          candidate?.content?.takeIf {
+            it.parts.isNotEmpty() || candidate.finishReason == FinishReason.STOP
+          },
         usageMetadata = response.usageMetadata,
         finishReason = finishReason,
         errorCode = finishReason?.takeIf { it != FinishReason.STOP }?.name,

core/src/commonTest/kotlin/com/google/adk/kt/models/LlmResponseTest.kt

@@ -25,6 +25,7 @@ import com.google.adk.kt.types.GenerateContentResponse
 import com.google.adk.kt.types.PromptFeedback
 import com.google.adk.kt.types.Role
 import kotlin.test.assertEquals
+import kotlin.test.assertNotNull
 import kotlin.test.assertNull
 import org.junit.Test
 import org.junit.runner.RunWith
@@ -89,6 +90,46 @@ class LlmResponseTest {
     assertEquals("Safety filter triggered", llmResponse.errorMessage)
   }
 
+  @Test
+  fun testContentlessCandidateHasNoContent() {
+    // No parts and not STOP: null content, not an empty Content (they serialize differently).
+    val response =
+      GenerateContentResponse(
+        candidates =
+          listOf(
+            Candidate(
+              content = Content(role = Role.MODEL, parts = emptyList()),
+              finishReason = FinishReason.SAFETY,
+            )
+          )
+      )
+
+    val llmResponse = LlmResponse.from(response)
+
+    assertNull(llmResponse.content)
+  }
+
+  @Test
+  fun testCreateStopWithEmptyPartsKeepsContent() {
+    // A candidate that finished normally keeps its (empty) content rather than dropping it.
+    val response =
+      GenerateContentResponse(
+        candidates =
+          listOf(
+            Candidate(
+              content = Content(role = Role.MODEL, parts = emptyList()),
+              finishReason = FinishReason.STOP,
+            )
+          )
+      )
+
+    val llmResponse = LlmResponse.from(response)
+
+    val content = assertNotNull(llmResponse.content)
+    assertEquals(0, content.parts.size)
+    assertNull(llmResponse.errorCode)
+  }
+
   @Test
   fun testCreateNoCandidates() {
     val response =