fix(template): fix the display of location and date

- extra space
- without the location, the comma still appeared
- docs(readme): add the optional comment for the sending location

lettre.typ

@@ -17,13 +17,23 @@
   show raw: set text(font: "Inconsolata")
   show heading: set block(above: 1.4em, below: 1em)
 
+  let formatLieuDate(lieu, date) = {
+    let isDate = date.trim().len() > 0;
+    let isLieu = lieu.trim().len() > 0;
+    if isLieu [#lieu]
+    else []
+    if isDate {
+      if isLieu [, ]
+      [le #date]
+    }
+  }
+
   grid(
     columns: (2fr, 1fr, 2fr),
     par(de),
     text[],
     text[
-      #lieu
-      #if date.trim().len() != 0 [, le #date ]
+      #formatLieuDate(lieu, date)
     ] + v(2em),
     text[],
     text[],

readme.md

@@ -19,7 +19,7 @@ parameters, with the [`lettre.typ`](./lettre.typ) file in the same folder:
   ],
   objet: "subject of the letter", // optional
   date: "date of sending", // optional
-  lieu: "location",
+  lieu: "location", // optional
   introduction: "opening",
   cloture: "closing",
   signature: "signature",