UnboundLocalError exception in `nikola deploy`
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### Environment

**Python Version:**

Python 3.7.9

**Nikola Version:**

Nikola v8.1.2 (installed from git master)

**Operating System:** 

FreeBSD 12.2-RELEASE amd64

### Description:

On running `nikola deploy` on a new site, after rsync completes, this exception is thrown:

```python
  File "[...]/nikola/nikola/plugins/command/deploy.py", line 99, in _execute
    self._emit_deploy_event(last_deploy, new_deploy, clean, undeployed_posts)
UnboundLocalError: local variable 'clean' referenced before assignment
```

I looked at the source and noticed that `clean` is only set inside a condition:

https://github.com/getnikola/nikola/blob/d0ef5b68d0667f635ecb2bc0d5b92e480b5e4b25/nikola/plugins/command/deploy.py#L50-L56

To try and fix it, I changed the source to:

```python
        if last_deploy is not None:
            last_deploy = dateutil.parser.parse(last_deploy)
            clean = False
        else:
            clean = True
```

That leads to another exception elsewhere:

```python
  File "[...]/nikola/nikola/plugins/command/deploy.py", line 131, in _emit_deploy_event
    if last_deploy.tzinfo is None:
AttributeError: 'NoneType' object has no attribute 'tzinfo'
```

Which was fixed by changing the source code at:

https://github.com/getnikola/nikola/blob/d0ef5b68d0667f635ecb2bc0d5b92e480b5e4b25/nikola/plugins/command/deploy.py#L98-L99

to:

```python
        new_deploy = datetime.utcnow()
        if last_deploy is None:
            last_deploy = new_deploy
        self._emit_deploy_event(last_deploy, new_deploy, clean, undeployed_posts)
```

After this the deploy command has worked as I expected.
diff --git a/nikola/plugins/command/deploy.py b/nikola/plugins/command/deploy.py
index 5273b58dd..11a0c9bf1 100644
--- a/nikola/plugins/command/deploy.py
+++ b/nikola/plugins/command/deploy.py
@@ -54,6 +54,8 @@ class CommandDeploy(Command):
         if last_deploy is not None:
             last_deploy = dateutil.parser.parse(last_deploy)
             clean = False
+        else:
+            clean = True
 
         if self.site.config['COMMENT_SYSTEM'] and self.site.config['COMMENT_SYSTEM_ID'] == 'nikolademo':
             self.logger.warning("\\nWARNING WARNING WARNING WARNING\\n"
@@ -96,6 +98,8 @@ class CommandDeploy(Command):
         self.logger.info("Successful deployment")
 
         new_deploy = datetime.utcnow()
+        if last_deploy is None:
+            last_deploy = new_deploy
         self._emit_deploy_event(last_deploy, new_deploy, clean, undeployed_posts)
 
         # Store timestamp of successful deployment
['nikola/plugins/command/deploy.py']