Make most of the fields in KeyinfoPubkey and KeyinfoSslcert models optional

The presence of the fields seems to be random and the only truly required field
is the actual public key/cert. Other fields are only for the information.

osc/obs_api/keyinfo_sslcert.py

@@ -8,7 +8,7 @@ class KeyinfoSslcert(XmlModel):
         xml_attribute=True,
     )
 
-    serial: str = Field(
+    serial: Optional[str] = Field(
         xml_attribute=True,
     )
 
@@ -16,23 +16,23 @@ class KeyinfoSslcert(XmlModel):
         xml_attribute=True,
     )
 
-    subject: str = Field(
+    subject: Optional[str] = Field(
         xml_attribute=True,
     )
 
-    algo: str = Field(
+    algo: Optional[str] = Field(
         xml_attribute=True,
     )
 
-    keysize: str = Field(
+    keysize: Optional[str] = Field(
         xml_attribute=True,
     )
 
-    begins: int = Field(
+    begins: Optional[int] = Field(
         xml_attribute=True,
     )
 
-    expires: int = Field(
+    expires: Optional[int] = Field(
         xml_attribute=True,
     )
 
@@ -46,10 +46,18 @@ class KeyinfoSslcert(XmlModel):
 
     def get_begins_str(self) -> str:
         import datetime
+
+        if self.begins is None:
+            return ""
+
         return datetime.datetime.fromtimestamp(self.begins).strftime("%Y-%m-%d %H:%M:%S")
 
     def get_expires_str(self) -> str:
         import datetime
+
+        if self.expires is None:
+            return ""
+
         return datetime.datetime.fromtimestamp(self.expires).strftime("%Y-%m-%d %H:%M:%S")
 
     def to_human_readable_string(self) -> str: