…onic-net#1923)

#### What I did
Fixing issue sonic-net#1908 

#### How I did it
- When the given patch produces empty-table, reject it since ConfigDb cannot show empty tables. Achieved that by:
  - Adding a validation step before patch-sorting
- The patch-sorter should not produce steps which result in empty-tables because it is not possible in ConfigDb, we should instead delete the whole table. Achieved that by:
  - Adding a new validator to reject moves producing empty tables
  - No need to add a generator for deleting the whole table, current generators take care of that. They do by the following:
    - The move to empty a table is rejected by `NoEmptyTableMoveValidator`
    - The previous rejected move is used to generate moves using `UpperLevelMoveExtender` which produces either
      - Remove move for parent -- This is good, we are removing the table
      - Replace move for parent -- This later on will be extended to a Delete move using `DeleteInsteadOfReplaceMoveExtender`

The generators/validators are documented in the `PatchSorter.py`, check the documentation out.

#### How to verify it
unit-test

#### Previous command output (if the output of a command-line utility has changed)
```sh
admin@vlab-01:~$ sudo config apply-patch empty-a-table.json-patch -v
Patch Applier: Patch application starting.
Patch Applier: Patch: [{"op": "replace", "path": "/DEVICE_METADATA", "value": {}}]
Patch Applier: Validating patch is not making changes to tables without YANG models.
Patch Applier: Getting current config db.
Patch Applier: Simulating the target full config after applying the patch.
Patch Applier: Validating target config according to YANG models.
... sonig-yang-mgmt logs
Patch Applier: Sorting patch updates.
... sonig-yang-mgmt logs
Patch Applier: The patch was sorted into 11 changes:
Patch Applier:   * [{"op": "remove", "path": "/DEVICE_METADATA/localhost/bgp_asn"}]
Patch Applier:   * [{"op": "remove", "path": "/DEVICE_METADATA/localhost/buffer_model"}]
Patch Applier:   * [{"op": "remove", "path": "/DEVICE_METADATA/localhost/default_bgp_status"}]
Patch Applier:   * [{"op": "remove", "path": "/DEVICE_METADATA/localhost/default_pfcwd_status"}]
Patch Applier:   * [{"op": "remove", "path": "/DEVICE_METADATA/localhost/deployment_id"}]
Patch Applier:   * [{"op": "remove", "path": "/DEVICE_METADATA/localhost/docker_routing_config_mode"}]
Patch Applier:   * [{"op": "remove", "path": "/DEVICE_METADATA/localhost/hostname"}]
Patch Applier:   * [{"op": "remove", "path": "/DEVICE_METADATA/localhost/hwsku"}]
Patch Applier:   * [{"op": "remove", "path": "/DEVICE_METADATA/localhost/mac"}]
Patch Applier:   * [{"op": "remove", "path": "/DEVICE_METADATA/localhost/platform"}]
Patch Applier:   * [{"op": "remove", "path": "/DEVICE_METADATA/localhost"}]
Patch Applier: Applying changes in order.
Patch Applier: Verifying patch updates are reflected on ConfigDB.
Failed to apply patch
Usage: config apply-patch [OPTIONS] PATCH_FILE_PATH
Try "config apply-patch -h" for help.

Error: After applying patch to config, there are still some parts not updated
admin@vlab-01:~$ 
```

The above error occurs because post the update, the whole `DEVICE_METADATA` table is deleted, not just showing as empty i.e. `"DEVICE_METADATA": {}`

#### New command output (if the output of a command-line utility has changed)
```
admin@vlab-01:~$ sudo config apply-patch empty-a-table.json-patch -v
Patch Applier: Patch application starting.
Patch Applier: Patch: [{"op": "replace", "path": "/DEVICE_METADATA", "value": {}}]
Patch Applier: Validating patch is not making changes to tables without YANG models.
Patch Applier: Getting current config db.
Patch Applier: Simulating the target full config after applying the patch.
Patch Applier: Validating target config does not have empty tables, since they do not show up in ConfigDb.
Failed to apply patch
Usage: config apply-patch [OPTIONS] PATCH_FILE_PATH
Try "config apply-patch -h" for help.

Error: Given patch is not valid because it will result in empty tables which is not allowed in ConfigDb. Table: DEVICE_METADATA
admin@vlab-01:~$ 
```