Fix #24599: Remove | Null from implicit BigDecimal conversion similar to BigInt

[noti0na1]

Fix #24599

We remove | Null from implicit BigDecimal conversion similar to BigInt.

// For the following function, both the parameter and the return type are non-nullable.
// However, if a null reference is passed explicitly, this method will still return null.
// We intentionally keep this signature to discourage passing nulls implicitly while
// preserving the previous behavior for backward compatibility.

/** Implicit conversion from `java.math.BigDecimal` to `scala.BigDecimal`. */
implicit def javaBigDecimal2bigDecimal(x: BigDec): BigDecimal = mapNull(x, apply(x))

/** Implicit conversion from `java.math.BigInteger` to `scala.BigInt`. */
implicit def javaBigInteger2bigInt(x: BigInteger): BigInt = mapNull(x, apply(x))

I have double-checked all other implicit functions in stdlib, and there is no nullable signature left anymore.