Write a method that calculates the area of a rectangle given its length and width. The method should return the area as a Decimal value.

Handle edge cases appropriately: return null for null inputs, and return 0 for zero or negative values.

Examples:

Decimal area = calculateRectangleArea(5.0, 3.0);
// Returns: 15.0

Decimal zeroArea = calculateRectangleArea(0.0, 5.0);
// Returns: 0.0

Decimal nullArea = calculateRectangleArea(null, 5.0);
// Returns: null