fix: prevent NPE in validateDouble/validateFloat when input is null

[anuragg-saxenaa]

Description

ConverterUtils.validateDouble(Double) and validateDouble(Float) accept boxed wrapper types but pass them directly to Double.isNaN() / Double.isFinite(), which take primitive double. Java auto-unboxes the wrapper before calling the method — if the wrapper is null, this auto-unboxing causes a NullPointerException.

This crash surfaces in practice when using Map<String, Double> or List<Double> DynamoDB attributes where one of the collection values is explicitly null.

Root Cause

// Before fix — crashes if input == null
public static void validateDouble(Double input) {
    Validate.isTrue(!Double.isNaN(input), "NaN is not supported by the default converters.");
    //                              ^^^^^ auto-unboxing: NPE if input == null
    Validate.isTrue(Double.isFinite(input), "Infinite numbers are not supported by the default converters.");
}

Fix

Added a null guard at the top of both validateDouble and validateFloat:

public static void validateDouble(Double input) {
    if (input == null) {
        return;  // null is a valid DynamoDB NULL; handled by converters separately
    }
    Validate.isTrue(!Double.isNaN(input), "NaN is not supported by the default converters.");
    Validate.isTrue(Double.isFinite(input), "Infinite numbers are not supported by the default converters.");
}

Tests

Added ConverterUtilsTest covering:

• null input does not throw (regression test for NullPointerException in DefaultAttributeConverterProvider when validating null Double values #6639)
• Finite values do not throw
• NaN throws IllegalArgumentException
• Infinite values throw IllegalArgumentException
• Same null/NaN coverage for validateFloat

Fixes #6639