[csharp][restsharp] guard throwOnAnyError against null FinalException

Polly's PolicyResult.FinalException is null when Outcome is Failure but
the failure type is ResultHandledByThisPolicy (e.g. a retry policy
configured with .HandleResult(...) that gives up after N retries).
Throwing it directly would NRE; fall back to InvalidOperationException
so the option still surfaces an error.

Also restores the trailing whitespace on the if line so existing
restsharp samples don't all need regenerating.

Author: mjansrud

modules/openapi-generator/src/main/resources/csharp/ApiClient.mustache

@@ -561,14 +561,14 @@ namespace {{packageName}}.Client
 
         private async Task<RestResponse<T>> DeserializeRestResponseFromPolicyAsync<T>(RestClient client, RestRequest request, PolicyResult<RestResponse> policyResult, CancellationToken cancellationToken = default)
         {
-            if (policyResult.Outcome == OutcomeType.Successful)
+            if (policyResult.Outcome == OutcomeType.Successful) 
             {
                 return await client.Deserialize<T>(policyResult.Result, cancellationToken).ConfigureAwait(false);
             }
             else
             {
                 {{#throwOnAnyError}}
-                throw policyResult.FinalException;
+                throw policyResult.FinalException ?? new InvalidOperationException("The retry policy failed without an exception.");
                 {{/throwOnAnyError}}
                 {{^throwOnAnyError}}
                 return new RestResponse<T>(request)

modules/openapi-generator/src/main/resources/csharp/ApiClient.v790.mustache

@@ -552,14 +552,14 @@ namespace {{packageName}}.Client
 
         private async Task<RestResponse<T>> DeserializeRestResponseFromPolicyAsync<T>(RestClient client, RestRequest request, PolicyResult<RestResponse> policyResult, CancellationToken cancellationToken = default)
         {
-            if (policyResult.Outcome == OutcomeType.Successful)
+            if (policyResult.Outcome == OutcomeType.Successful) 
             {
                 return await client.Deserialize<T>(policyResult.Result, cancellationToken);
             }
             else
             {
                 {{#throwOnAnyError}}
-                throw policyResult.FinalException;
+                throw policyResult.FinalException ?? new InvalidOperationException("The retry policy failed without an exception.");
                 {{/throwOnAnyError}}
                 {{^throwOnAnyError}}
                 return new RestResponse<T>(request)