Add coverage for valid maxStalenessSeconds with no viable servers

JAVA-3760

Author: jstewart148

driver-core/src/main/com/mongodb/TaggableReadPreference.java

@@ -255,10 +255,12 @@ protected List<ServerDescription> selectFreshServers(final ClusterDescription cl
             }
         } else {
             ServerDescription mostUpToDateSecondary = findMostUpToDateSecondary(clusterDescription);
-            for (ServerDescription cur : servers) {
-                if (getLastWriteDateNonNull(mostUpToDateSecondary).getTime() - getLastWriteDateNonNull(cur).getTime()
-                        + heartbeatFrequencyMS <= maxStaleness) {
-                    freshServers.add(cur);
+            if (mostUpToDateSecondary != null) {
+                for (ServerDescription cur : servers) {
+                    if (getLastWriteDateNonNull(mostUpToDateSecondary).getTime() - getLastWriteDateNonNull(cur).getTime()
+                            + heartbeatFrequencyMS <= maxStaleness) {
+                        freshServers.add(cur);
+                    }
                 }
             }
         }
@@ -293,9 +295,6 @@ private ServerDescription findMostUpToDateSecondary(final ClusterDescription clu
                 }
             }
         }
-        if (mostUpdateToDateSecondary == null) {
-            throw new MongoInternalException("Expected at least one secondary in cluster description: " + clusterDescription);
-        }
         return mostUpdateToDateSecondary;
     }
 

driver-core/src/test/resources/max-staleness/server_selection/ReplicaSetNoPrimary/MaxStalenessTooSmall.json

@@ -0,0 +1,20 @@
+{
+  "topology_description": {
+    "type": "ReplicaSetNoPrimary",
+    "servers": [
+      {
+        "address": "a:27017",
+        "type": "Unknown"
+      },
+      {
+        "address": "b:27017",
+        "type": "Unknown"
+      }
+    ]
+  },
+  "read_preference": {
+    "mode": "Nearest",
+    "maxStalenessSeconds": 1
+  },
+  "error": true
+}

driver-core/src/test/resources/max-staleness/server_selection/ReplicaSetNoPrimary/NoKnownServers.json

@@ -14,7 +14,8 @@
   },
   "read_preference": {
     "mode": "Nearest",
-    "maxStalenessSeconds": 1
+    "maxStalenessSeconds": 90
   },
-  "error": true
+  "suitable_servers": [],
+  "in_latency_window": []
 }

driver-core/src/test/resources/max-staleness/server_selection/Unknown/SmallMaxStaleness.json

@@ -5,7 +5,8 @@
     "servers": [
       {
         "address": "a:27017",
-        "type": "Unknown"
+        "type": "Unknown",
+        "maxWireVersion": 5
       }
     ]
   },