This pull request updates how null/default values for decimal types are handled in both TableWriter and WalWriter, switching from hardcoded primitive constants to centralized constants in the Decimals class. It also adds a new test to ensure that decimal columns default to null values when not provided. These changes improve consistency, maintainability, and correctness in handling decimal nulls.

Decimal null value handling improvements:

• Replaced hardcoded primitive constants (e.g., Byte.MIN_VALUE, Short.MIN_VALUE, etc.) with Decimals class constants (e.g., Decimals.DECIMAL8_NULL, Decimals.DECIMAL16_NULL, etc.) for all decimal types in the configureNullSetters methods of both TableWriter and WalWriter. This ensures a single source of truth for decimal null representations and reduces the risk of errors if null values change in the future. [1] [2]
• Added imports for the Decimals class in both TableWriter.java and WalWriter.java to support the above change. [1] [2]

Testing improvements:

• Added a new test testDecimalDefaultValues in LineHttpSenderTest.java to verify that decimal columns are set to their default null values when not explicitly provided in the input. This test creates a table with various decimal columns, inserts a row without specifying decimal values, and asserts that the resulting row has nulls in all decimal columns.