always consider empty strings as no value given

interfaces/IDocProps.ts

@@ -1,15 +1,19 @@
 export interface IDocProps {
-  template: string
-  subject: string
-  body: string
   address: string
-  date?: string
-  opening?: string
-  closing?: string
-  yourRef?: string
-  yourMail?: string
-  myRef?: string
-  customer?: string
-  invoice?: string
-  specialMail?: string
+  body: string
+  closing: string
+  customer: string
+  date: string
+  enclosing: string
+  invoice: string
+  myRef: string
+  opening: string
+  ps: string
+  signature: string
+  specialMail: string
+  subject: string
+  template: string
+  yourMail: string
+  yourRef: string
+  yourRefName: string
 }