fix: prevent NPE

fix: do not use java21 features

Author: KaiSuchomel

model-providers/google/gemini/gemini-common/runtime/src/main/java/io/quarkiverse/langchain4j/gemini/common/GenerateContentResponseHandler.java

@@ -20,10 +20,14 @@ public static String getText(GenerateContentResponse response) {
         }
 
         StringBuilder text = new StringBuilder();
-        List<GenerateContentResponse.Candidate.Part> parts = response.candidates().get(0).content().parts();
-        if (parts != null && !parts.isEmpty()) {
-            for (GenerateContentResponse.Candidate.Part part : parts) {
-                text.append(part.text());
+        if (response.candidates() != null && !response.candidates().isEmpty()) {
+            if (response.candidates().get(0).content() != null) {
+                List<GenerateContentResponse.Candidate.Part> parts = response.candidates().get(0).content().parts();
+                if (parts != null && !parts.isEmpty()) {
+                    for (GenerateContentResponse.Candidate.Part part : parts) {
+                        text.append(part.text());
+                    }
+                }
             }
         }
         return text.toString();