fix(dynamodb): BETWEEN filter incorrectly excludes attribute value 0

[Sohel2309]

Bug

DynamoDB BETWEEN filtering incorrectly excludes items whose attribute
value is exactly 0, even when 0 is within the specified range.

Root cause

FuncBetween.expr() checked the tested attribute using bare Python
truthiness (and attr). Since Decimal("0") is falsy, an attribute
whose value is zero was incorrectly treated as having no value.

The start and end operands already use zero-safe checks for this
exact case.

Fix

Apply the same zero-safe value check to the attribute being tested.

Testing

• Added regression coverage for attribute value 0
• Confirmed the regression test fails on the unfixed code
• Confirmed the targeted tests pass after the fix
• 3/3 relevant tests pass
• git diff --check passes