Don't crash RTDB SDK if version file is empty (#5719)

Author: schmidt-sebastian

Firebase/Database/CHANGELOG.md

@@ -1,3 +1,7 @@
+# v6.2.2
+- [fixed] Addressed crash that prevented the SDK from opening when the versioning file was
+  corrupted. (#5686)
+
 # v6.2.1
 - [fixed] Fixed documentation typos. (#5406, #5418)
 

Firebase/Database/Persistence/FLevelDBStorageEngine.m

@@ -132,6 +132,9 @@ - (void)runMigration {
         }
     } else if ([oldVersion isEqualToString:kFPersistenceVersion]) {
         // Everythings fine no need for migration
+    } else if ([oldVersion length] == 0) {
+        FFWarn(@"I-RDB076036",
+               @"Version file empty. Assuming database version 1.");
     } else {
         // If we add more versions in the future, we need to run migration here
         [NSException raise:NSInternalInconsistencyException