Add null-safe comparisons in model Equals methods

Description

Several model classes in the SDK contain Equals methods that may throw NullReferenceException when comparing nullable properties. The current pattern uses:

this.Property == input.Property || this.Property.Equals(input.Property)

This will throw if this.Property is null and input.Property is non-null.

Affected Classes

Known affected models:

• WriteRequestWrites (OnDuplicate property)
• WriteRequestDeletes (OnMissing property)

All other model classes with nullable properties should be audited for similar issues.

Proposed Fix

Replace unsafe comparisons with null-safe alternatives:

object.Equals(this.Property, input.Property)

Or:

this.Property == input.Property || (this.Property \!= null && this.Property.Equals(input.Property))

Context

Identified during review of PR #120.

Referenced by: @rhamzeh

• PR: https://github.com/openfga/dotnet-sdk/pull/120
• Discussion: https://github.com/openfga/dotnet-sdk/pull/120#discussion_r2380256985

Inspect the Equals methods in WriteRequestWrites and WriteRequestDeletes first, focusing on OnDuplicate and OnMissing, then audit other model classes for the same nullable-property pattern. Confirm that comparisons where either property is null no longer throw a NullReferenceException and that existing equality behavior remains intact.