Fix exception when estimating token count of tool execution request

[jpenilla]

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

• 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
• I have added an example in the examples repo (only for "big" features)
• I have added/updated Spring Boot starter(s) (if applicable)

[dliubarskyi]

@jpenilla thank you!