models/user.py: properly guard plain_text_password property

Resolves the following issue, which occurs with force_otp enabled
and OAuth authentication sources:

File "/srv/powerdnsadmin/powerdnsadmin/models/user.py", line 481, in update_profile
  "utf-8") if self.plain_text_password else user.password
AttributeError: 'User' object has no attribute 'plain_text_password'

powerdnsadmin/models/user.py

@@ -107,7 +107,7 @@ class User(db.Model):
 
     def check_password(self, hashed_password):
         # Check hashed password. Using bcrypt, the salt is saved into the hash itself
-        if (self.plain_text_password):
+        if hasattr(self, "plain_text_password"):
             return bcrypt.checkpw(self.plain_text_password.encode('utf-8'),
                                   hashed_password.encode('utf-8'))
         return False
@@ -423,7 +423,7 @@ class User(db.Model):
                 name='Administrator').first().id
 
         self.password = self.get_hashed_password(
-            self.plain_text_password) if self.plain_text_password else '*'
+            self.plain_text_password) if hasattr(self, "plain_text_password") else '*'
 
         if self.password and self.password != '*':
             self.password = self.password.decode("utf-8")
@@ -459,7 +459,7 @@ class User(db.Model):
         user.email = self.email
 
         # store new password hash (only if changed)
-        if self.plain_text_password:
+        if hasattr(self, "plain_text_password"):
             user.password = self.get_hashed_password(
                 self.plain_text_password).decode("utf-8")
 
@@ -478,7 +478,7 @@ class User(db.Model):
         user.lastname = self.lastname if self.lastname else user.lastname
         user.password = self.get_hashed_password(
             self.plain_text_password).decode(
-                "utf-8") if self.plain_text_password else user.password
+                "utf-8") if hasattr(self, "plain_text_password") else user.password
 
         if self.email:
             # Can not update to a new email that