fix(spring-ai-openai): Prevent out-of-range errors when merging tool calls.

Fixes #4229 Signed-off-by: xfl12345 <17960863+xfl12345@users.noreply.github.com> (cherry picked from commit 6697589)

Author: xfl12345

models/spring-ai-openai/src/main/java/org/springframework/ai/openai/api/OpenAiStreamFunctionCallingHelper.java

@@ -112,13 +112,13 @@ private ChatCompletionMessage merge(ChatCompletionMessage previous, ChatCompleti
 
 List<ToolCall> toolCalls = new ArrayList<>();
 ToolCall lastPreviousTooCall = null;
-if (previous.toolCalls() != null) {
+if (previous.toolCalls() != null && !previous.toolCalls().isEmpty()) {
 lastPreviousTooCall = previous.toolCalls().get(previous.toolCalls().size() - 1);
 if (previous.toolCalls().size() > 1) {
 toolCalls.addAll(previous.toolCalls().subList(0, previous.toolCalls().size() - 1));
 }
 }
-if (current.toolCalls() != null && current.toolCalls().size() > 0) {
+if (current.toolCalls() != null && !current.toolCalls().isEmpty()) {
 if (current.toolCalls().size() > 1) {
 throw new IllegalStateException("Currently only one tool call is supported per message!");
 }