fix(parser): emit decimal bigint for zero-valued non-decimal literals in ESTree - #18203

What

Fix the bigint field produced by the ESTree plugin for zero-valued BigInt literals written in a non-decimal radix (hex/octal/binary). 0x0n now yields bigint: "0" instead of "0x0".

Why

parseBigIntLiteral computed the field with:

node.bigint = String(node.value || value);

The || value fallback exists only for environments where the BigInt could not be constructed (node.value === null), in which case the raw source string is used. But 0n is falsy, so any zero-valued BigInt — regardless of radix — also fell through to the raw source string, which still carries the radix prefix. As a result 0x0n serialized to "0x0" while 0xFFn correctly serialized to "255", and the ESTree spec defines bigint as the string representation of the value in base 10.

How

Replace the falsy check with an explicit null check so real BigInt values (including 0n) always go through the decimal String(node.value) path, and only the unsupported-BigInt case uses the raw source string:

node.bigint = node.value == null ? value : String(node.value);

Test

Added two ESTree fixtures under test/fixtures/estree/bigInt/: hex-zero (0x0n → bigint: "0", the regression) and hex-nonzero (0xFFn → bigint: "255", showing consistency). The hex-zero fixture fails before the change ("0x0" != "0") and passes after.