fix(pageserver): report synthetic size = 1 if all tls offloaded (2) (#11731)

## Problem

https://github.com/neondatabase/neon/pull/11648 did this for resident
size instead of synthetic size.

## Summary of changes

Report synthetic_size == 1 if all timelines are offloaded.

Signed-off-by: Alex Chi Z <chi@neon.tech>

libs/tenant_size_model/src/calculation.rs

@@ -77,7 +77,9 @@ impl StorageModel {
         }
 
         SizeResult {
-            total_size,
+            // If total_size is 0, it means that the tenant has all timelines offloaded; we need to report 1
+            // here so that the data point shows up in the s3 files.
+            total_size: total_size.max(1),
             segments: segment_results,
         }
     }