db: prevent dbulog writing to file if lazy loaded

If DatabaseULog is lazy loaded it doesn't have data, so it will fail if
trying to write to file.

pyulog/db.py

@@ -170,6 +170,7 @@ def __init__(self, db_handle, primary_key=None, log_file=None, lazy=True, **kwar
 
         self._pk = primary_key
         self._db = db_handle
+        self._lazy_loaded = lazy
         if log_file is not None:
             self._sha256sum = DatabaseULog.calc_sha256sum(log_file)
 
@@ -187,6 +188,11 @@ def __eq__(self, other):
             return other.__eq__(self)
         return super().__eq__(other)
 
+    def write_ulog(self, path):
+        if self._lazy_loaded:
+            raise ValueError('Cannot write after lazy load because it has no datasets.')
+        super().write_ulog(path)
+
     @property
     def primary_key(self):
         '''The primary key of the ulog, pointing to the correct "ULog" row in the database.'''
@@ -400,6 +406,7 @@ def load(self, lazy=True):
                 self._changed_parameters.append((timestamp, key, value))
 
             cur.close()
+        self._lazy_loaded = lazy
 
     def get_dataset(self, name, multi_instance=0, lazy=False, db_cursor=None, caching=True):
         '''