Generate unique email aliases by appending incremental numbers to a base email until a unique one is found. Check against a provided list of existing emails and return the first available alias. Return null when the base email is null or empty.

Examples:

Set<String> existing = new Set<String>{'user@domain.com', 'user1@domain.com'};
String alias = generateUniqueAlias('user@domain.com', existing);
// Results: 'user2@domain.com'

Set<String> empty = new Set<String>();
String original = generateUniqueAlias('test@example.com', empty);
// Results: 'test@example.com'

Hints:

• Validate the base email parameter before processing
• Start by checking if the original email is already unique
• Use a counter to generate sequential numbered aliases
• Split the email at the @ symbol to insert numbers before the domain