fix(zentao): fix parsing errors for date related fields on zentao v18.10 (#7051) (#7054)

Co-authored-by: Lynwee <linwei.hou@merico.dev>
diff --git a/backend/core/models/common/iso8601time.go b/backend/core/models/common/iso8601time.go
index 6880821..512a220 100644
--- a/backend/core/models/common/iso8601time.go
+++ b/backend/core/models/common/iso8601time.go
@@ -97,6 +97,9 @@
 // UnmarshalJSON FIXME ...
 func (jt *Iso8601Time) UnmarshalJSON(b []byte) error {
 	timeString := string(b)
+	if timeString == `""` {
+		return nil
+	}
 	if timeString == "null" {
 		return nil
 	}
diff --git a/backend/core/models/migrationscripts/archived/iso8601time.go b/backend/core/models/migrationscripts/archived/iso8601time.go
index 84af885..a2597fd 100644
--- a/backend/core/models/migrationscripts/archived/iso8601time.go
+++ b/backend/core/models/migrationscripts/archived/iso8601time.go
@@ -100,6 +100,9 @@
 	if timeString == "null" {
 		return nil
 	}
+	if timeString == `""` {
+		return nil
+	}
 	if strings.Contains(timeString, "0000-00-00") {
 		return nil
 	}