Don't return a 204 if there's no historical data (#17935)

* don't return a 204 if there's no historical data

* add changelog

changelog/17935.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+core/activity: return partial month counts when querying a historical date range and no historical data exists.
+```

vault/activity_log.go

@@ -1546,10 +1546,19 @@ func (a *ActivityLog) handleQuery(ctx context.Context, startTime, endTime time.T
 			return nil, err
 		}
 		if storedQuery == nil {
-			return nil, nil
+			// If the storedQuery is nil, that means there's no historical data to process. But, it's possible there's
+			// still current month data to process, so rather than returning a 204, let's proceed along like we're
+			// just querying the current month.
+			storedQuery = &activity.PrecomputedQuery{
+				StartTime:  startTime,
+				EndTime:    endTime,
+				Namespaces: make([]*activity.NamespaceRecord, 0),
+				Months:     make([]*activity.MonthRecord, 0),
+			}
 		}
 		pq = storedQuery
 	}
+
 	// Calculate the namespace response breakdowns and totals for entities and tokens from the initial
 	// namespace data.
 	totalEntities, totalTokens, byNamespaceResponse, err := a.calculateByNamespaceResponseForQuery(ctx, pq.Namespaces)
@@ -1584,6 +1593,7 @@ func (a *ActivityLog) handleQuery(ctx context.Context, startTime, endTime time.T
 		// Add the current month's namespace data the precomputed query namespaces
 		byNamespaceResponse = append(byNamespaceResponse, byNamespaceResponseCurrent...)
 	}
+
 	// Sort clients within each namespace
 	a.sortALResponseNamespaces(byNamespaceResponse)
 
@@ -1597,11 +1607,13 @@ func (a *ActivityLog) handleQuery(ctx context.Context, startTime, endTime time.T
 		if err != nil {
 			return nil, err
 		}
+
 		// Add the namespace attribution for the current month to the newly computed current month value. Note
 		// that transformMonthBreakdowns calculates a superstruct of the required namespace struct due to its
 		// primary use-case being for precomputedQueryWorker, but we will reuse this code for brevity and extract
 		// the namespaces from it.
 		currentMonthNamespaceAttribution := a.transformMonthBreakdowns(partialByMonth)
+
 		// Ensure that there is only one element in this list -- if not, warn.
 		if len(currentMonthNamespaceAttribution) > 1 {
 			a.logger.Warn("more than one month worth of namespace and mount attribution calculated for "+