fix getPreviousPosition npe (apache#11621)

Fixes apache#11619

### Motivation
There's a chance that headMap might be empty even if a empty check added. because remove would be executed in ledgers. after the empty check between line 3214 and line 3219 in following code snippet:

headMap is a view of ledgers

pulsar/managed-ledger/src/main/java/org/apache/bookkeeper/mledger/impl/ManagedLedgerImpl.java

Lines 3206 to 3222 in 31231d6
```
 NavigableMap<Long, LedgerInfo> headMap = ledgers.headMap(position.getLedgerId(), false); 
  
 if (headMap.isEmpty()) { 
     // There is no previous ledger, return an invalid position in the current ledger 
     return PositionImpl.get(position.getLedgerId(), -1); 
 } 
  
 // We need to find the most recent non-empty ledger 
 for (long ledgerId : headMap.descendingKeySet()) { 
     LedgerInfo li = headMap.get(ledgerId); 
     if (li.getEntries() > 0) { 
         return PositionImpl.get(li.getLedgerId(), li.getEntries() - 1); 
     } 
 } 
  
 // in case there are only empty ledgers, we return a position in the first one 
 return PositionImpl.get(headMap.firstEntry().getKey(), -1); 
```

### Modifications
Add a null checker of headMap.firstEntry

Verifying this change
Make sure that the change passes the CI checks.
This change is a trivial rework / code cleanup without any test coverage.

Co-authored-by: gavingaozhangmin <[email protected]>

managed-ledger/src/main/java/org/apache/bookkeeper/mledger/impl/ManagedLedgerImpl.java

@@ -48,6 +48,7 @@
 import java.util.Queue;
 import java.util.Random;
 import java.util.Set;
+import java.util.TreeMap;
 import java.util.UUID;
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.CompletionException;
@@ -3205,21 +3206,22 @@ public PositionImpl getPreviousPosition(PositionImpl position) {
         // The previous position will be the last position of an earlier ledgers
         NavigableMap<Long, LedgerInfo> headMap = ledgers.headMap(position.getLedgerId(), false);
 
-        if (headMap.isEmpty()) {
+        final Map.Entry<Long, LedgerInfo> firstEntry = headMap.firstEntry();
+        if (firstEntry == null) {
             // There is no previous ledger, return an invalid position in the current ledger
             return PositionImpl.get(position.getLedgerId(), -1);
         }
 
         // We need to find the most recent non-empty ledger
         for (long ledgerId : headMap.descendingKeySet()) {
             LedgerInfo li = headMap.get(ledgerId);
-            if (li.getEntries() > 0) {
+            if (li != null && li.getEntries() > 0) {
                 return PositionImpl.get(li.getLedgerId(), li.getEntries() - 1);
             }
         }
 
         // in case there are only empty ledgers, we return a position in the first one
-        return PositionImpl.get(headMap.firstEntry().getKey(), -1);
+        return PositionImpl.get(firstEntry.getKey(), -1);
     }
 
     /**