Fix validation error for ImagePullPolicy in charms

Change-Id: I00711202fdbacd73e9205dd3fc34424893440be5
Signed-off-by: sousaedu <eduardo.sousa@canonical.com>
diff --git a/installers/charm/kafka-exporter/src/charm.py b/installers/charm/kafka-exporter/src/charm.py
index 2147781..7eaa2a0 100755
--- a/installers/charm/kafka-exporter/src/charm.py
+++ b/installers/charm/kafka-exporter/src/charm.py
@@ -52,7 +52,7 @@
     ingress_class: Optional[str]
     ingress_whitelist_source_range: Optional[str]
     tls_secret_name: Optional[str]
-    image_pull_policy: Optional[str]
+    image_pull_policy: str
 
     @validator("site_url")
     def validate_site_url(cls, v):