Fix #913: FileSystemDocumentLoader: ignore empty/blank documents, improved error/warn messages

[langchain4j]

Context

When loading with FileSystemDocumentLoader and it encounters an empty file, a WARN message in the logs looks like an exception and is confusing.
See #913

Change

• Ignore empty/blank documents when loading multiple files with DocumentIsBlankException
• Changed log message to look not like an exception

Checklist

Before submitting this PR, please check the following points:

• I have added unit and integration tests for my change
• All unit and integration tests in the module I have added/changed are green
• All unit and integration tests in the core and main modules are green
• I have added/updated the documentation
• I have added an example in the examples repo (only for "big" features)
• I have added my new module in the BOM (only when a new module is added)

Checklist for adding new embedding store integration

• I have added a {NameOfIntegration}EmbeddingStoreIT that extends from either EmbeddingStoreIT or EmbeddingStoreWithFilteringIT