Refactor: use ternary operators for conciseness

This PR refactors a few conditional return statements to use ternary operators in order to improve readability and conciseness.

Changes:

• Replaced simple if-else statements in methods like Mono#flux(), Mono#filter() with equivalent ternary expressions.
• Ensured that the logic remains unchanged and safe (e.g., instanceof checks and casting).

Rationale:

• Improves brevity and readability where the condition and branches are simple.
• Helps align the style with similar concise expressions already present in the codebase.

Let me know if there's any concern about the use of ternary in these contexts.