FMDatabase: fixes static analyzer warning, API Misuse (Apple)

FMDatabase.m:207:5: Key argument to 'setObject:forKey:' cannot be nil

src/FMDatabase.m

@@ -704,7 +704,7 @@ - (FMResultSet *)executeQuery:(NSString *)sql withArgumentsInArray:(NSArray*)arr
         statement = [[FMStatement alloc] init];
         [statement setStatement:pStmt];
         
-        if (_shouldCacheStatements) {
+        if (_shouldCacheStatements && sql) {
             [self setCachedStatement:statement forQuery:sql];
         }
     }