Optimization: Skip `CalDateTime.Add(Duration)` for Empty Durations

[axunonb]

Summary

During recurrence evaluation, CalDateTime.Add(Duration) may be invoked with an empty duration. This introduces unnecessary overhead, especially in high-frequency scenarios like GetOccurrences.

Proposed Optimization

Introduce a shortcut to bypass the operation when the duration is empty:

if (d.IsEmpty) return this;

Benchmark Results

Environment:

• BenchmarkDotNet v0.15.0
• Windows 11 (24H2, HudsonValley)
• Intel Core i7-13700K, 24 logical cores
• .NET SDK 9.0.306, Host: .NET 8.0.21
• Toolchain: InProcessNoEmitToolchain

Without Shortcut

Method	Mean	Error	StdDev	Gen0	Gen1	Allocated
SerializeCalendar	7.457 μs	0.0559 μs	0.0495 μs	1.5640	0.0381	24.05 KB
DeserializeCalendar	15.619 μs	0.0640 μs	0.0599 μs	3.1128	0.1526	47.89 KB
GetOccurrences	924.022 μs	9.4472 μs	7.8888 μs	159.1797	99.6094	2438.35 KB

With Shortcut

Method	Mean	Error	StdDev	Gen0	Gen1	Allocated
SerializeCalendar	7.517 μs	0.0412 μs	0.0386 μs	1.5640	0.0381	24.05 KB
DeserializeCalendar	15.349 μs	0.0581 μs	0.0485 μs	3.1128	0.1526	47.89 KB
GetOccurrences	831.747 μs	5.5135 μs	4.8875 μs	149.4141	98.6328	2297.72 KB

Impact

• ~10% improvement in GetOccurrences performance
• Reduced allocations and GC pressure
• No behavioral change - pure optimization