Error when reading milvus data for empty collection

[FT-JMendyk]

Issue description

When trying to read an empty collection, milvusdm fails saying that total_vectors variable is referenced before assignment. This error origins from get_files_data function in read_milvus_data.py: https://github.com/milvus-io/milvus-tools/blob/41143e5f4f5ef3693e4abd903e6b443b98804cb4/pymilvusdm/core/read_milvus_data.py#L55-L80

When either segment_list or row_list is empty, the for loop won't run and thus an attempt is made to return total_vectors and total_ids which haven't been initialized.

I've created a PR with a simple fix/workaround for this issue: https://github.com/milvus-io/milvus-tools/pull/33

Reproduction steps

Using Milvus 1.1.1, pymilvus==1.1.1 and pymilvusdm==2.0.

1. Create collection using pymilvus:

   _DIM = 8
   from milvus import Milvus, IndexType, MetricType, Status
   milvus = Milvus('127.0.0.1', '19530')
   collection_name = 'example_collection'
   param = { 'collection_name': collection_name, 'dimension': _DIM }
   milvus.create_collection(param)
   milvus.flush([collection_name])

2. Prepare configuration YAML M2H.yml:

   M2H:
   milvus_version: 1.1.1
   source_milvus_path: '<SOURCE_MILVUS_PATH>'
   mysql_parameter:
   host: '127.0.0.1'
   user: 'root'
   port: 3306
   password: 'password'
   database: 'milvus'
   source_collection:
   example_collection:
   data_dir: 'backup'

3. Call milvusdm --yaml M2H.yml:

   <TIMESTAMP> ｜ INFO ｜ milvus_to_hdf5.py ｜ read_milvus_data ｜ 49 ｜ Ready to read all data of collection: example_collection/partitions: [None]
   0%|                                                                                                                      | 0/1 [00:00<?, ?it/s]
   <TIMESTAMP>｜ ERROR ｜ milvus_to_hdf5.py ｜ read_milvus_data ｜ 56 ｜ Error with: local variable 'total_vectors' referenced before assignment

Same error happens when non-default partition is used and contains some vectors, while the default partition stays empty.

[shiyu22]

Closed by #33 .

Welcome to be a contributor to Milvus-tools :)