Write a method that calculates the number of business days (Monday through Friday) between two dates, inclusive of both start and end dates if they are business days.

Exclude weekends (Saturday and Sunday) from the count. Handle edge cases like null inputs and invalid date ranges appropriately.

Examples:

Integer days = calculateBusinessDays(Date.newInstance(2024, 1, 1), Date.newInstance(2024, 1, 5));
// Returns: 5 (Monday to Friday)

Integer weekend = calculateBusinessDays(Date.newInstance(2024, 1, 6), Date.newInstance(2024, 1, 7));
// Returns: 0 (Saturday to Sunday)

Integer invalid = calculateBusinessDays(Date.newInstance(2024, 1, 5), Date.newInstance(2024, 1, 1));
// Returns: 0 (end date before start date)
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