Fix validation error for ImagePullPolicy in charms

Change-Id: I00711202fdbacd73e9205dd3fc34424893440be5
Signed-off-by: sousaedu <eduardo.sousa@canonical.com>
diff --git a/installers/charm/nbi/src/charm.py b/installers/charm/nbi/src/charm.py
index 0e36073..1460459 100755
--- a/installers/charm/nbi/src/charm.py
+++ b/installers/charm/nbi/src/charm.py
@@ -61,7 +61,7 @@
     ingress_whitelist_source_range: Optional[str]
     tls_secret_name: Optional[str]
     mongodb_uri: Optional[str]
-    image_pull_policy: Optional[str]
+    image_pull_policy: str
 
     @validator("auth_backend")
     def validate_auth_backend(cls, v):