nixos/dovecot: Make quoted-strings regexp compatible with macOS (cf: N…

…ixOS/nix#1537)

Also avoid trimming single-line string values unnecessarily.

nixos/modules/services/mail/dovecot.nix

@@ -32,7 +32,7 @@ let
         # Find index of last non-empty line in line list
         lastContentIndex = length lines - (findFirstIndex (
           line: match "[[:space:]]*" line == null  # <=> Line is not empty
-        ) (length lines) (reverseList lines));
+        ) (length lines - 1) (reverseList lines));
         # Drop empty trailing lines from line list
         trimmedLines = sublist 0 lastContentIndex lines;
       in
@@ -47,7 +47,7 @@ let
   escapeDovecotString = string:
     let
       escapedString = escape ["\\" "\""] string;
-    in   if match ''[[:space:]]*<.*|.*[[:space:]#"\].*|'' string != null
+    in   if match ''[[:space:]]*<.*|.*[[:space:]#"\\].*|^$'' string != null
     then "\"${formatDovecotMultiline escapedString}\""
     else string;