Fix exception when estimating token count of tool execution request (#85)

## Issue
Closes langchain4j/langchain4j#3409

The HuggingFaceTokenCountEstimator does not properly check for null text
in AiMessages which may be tool execution requests. This results in a
stacktrace similar to the following:
```
java.util.concurrent.CompletionException: java.lang.NullPointerException: text cannot be null
        at java.base/java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:315)
        at java.base/java.util.concurrent.CompletableFuture.uniApplyNow(CompletableFuture.java:687)
        at java.base/java.util.concurrent.CompletableFuture.uniApplyStage(CompletableFuture.java:662)
        at java.base/java.util.concurrent.CompletableFuture.thenApply(CompletableFuture.java:2200)
        ...
Caused by: java.lang.NullPointerException: text cannot be null
        at ai.djl.huggingface.tokenizers.HuggingFaceTokenizer.encode(HuggingFaceTokenizer.java:237)
        at dev.langchain4j.model.embedding.onnx.HuggingFaceTokenCountEstimator.estimateTokenCountInText(HuggingFaceTokenCountEstimator.java:99)
        at dev.langchain4j.model.embedding.onnx.HuggingFaceTokenCountEstimator.estimateTokenCountInMessage(HuggingFaceTokenCountEstimator.java:110)
        at dev.langchain4j.model.embedding.onnx.HuggingFaceTokenCountEstimator.estimateTokenCountInMessages(HuggingFaceTokenCountEstimator.java:122)
        at dev.langchain4j.memory.chat.TokenWindowChatMemory.ensureCapacity(TokenWindowChatMemory.java:95)
        at dev.langchain4j.memory.chat.TokenWindowChatMemory.add(TokenWindowChatMemory.java:71)
        at dev.langchain4j.service.tool.ToolService.executeInferenceAndToolsLoop(ToolService.java:128)
        ...
```

## Change
We check for null text and return 0 for the estimated token count.

## General checklist
- [X] There are no breaking changes
- [ ] I have added unit and/or integration tests for my change
- [ ] The tests cover both positive and negative cases
- [ ] I have manually run all the unit and integration tests in the
module I have added/changed, and they are all green
- [ ] I have added/updated the
[documentation](https://github.com/langchain4j/langchain4j/tree/main/docs/docs)
- [ ] I have added an example in the [examples
repo](https://github.com/langchain4j/langchain4j-examples) (only for
"big" features)
- [ ] I have added/updated [Spring Boot
starter(s)](https://github.com/langchain4j/langchain4j-spring) (if
applicable)

Co-authored-by: Dmytro Liubarskyi <langchain4j@gmail.com>

Author: jpenilla

langchain4j-embeddings/src/main/java/dev/langchain4j/model/embedding/onnx/HuggingFaceTokenCountEstimator.java

@@ -107,7 +107,7 @@ public int estimateTokenCountInMessage(ChatMessage message) {
         } else if (message instanceof UserMessage userMessage) {
             return estimateTokenCountInText(userMessage.singleText());
         } else if (message instanceof AiMessage aiMessage) {
-            return estimateTokenCountInText(aiMessage.text());
+            return aiMessage.text() == null ? 0 : estimateTokenCountInText(aiMessage.text());
         } else if (message instanceof ToolExecutionResultMessage toolExecutionResultMessage) {
             return estimateTokenCountInText(toolExecutionResultMessage.text());
         } else {