fixes a critical times.nim bug reported on IRC [backport] (#13216)

lib/pure/strformat.nim

@@ -717,6 +717,9 @@ when isMainModule:
   var tm = fromUnix(0)
   discard &"{tm}"
 
+  var noww = now()
+  check &"{noww}", $noww
+
   # Unicode string tests
   check &"""{"αβγ"}""", "αβγ"
   check &"""{"αβγ":>5}""", "  αβγ"

lib/pure/times.nim

@@ -2339,9 +2339,10 @@ proc format*(dt: DateTime, f: static[string]): string {.raises: [].} =
   const f2 = initTimeFormat(f)
   result = dt.format(f2)
 
-template formatValue*(result: var string; value: DateTime, specifier: string) =
+proc formatValue*(result: var string; value: DateTime, specifier: string) =
   ## adapter for strformat. Not intended to be called directly.
-  result.add format(value, specifier)
+  result.add format(value,
+    if specifier.len == 0: "yyyy-MM-dd'T'HH:mm:sszzz" else: specifier)
 
 proc format*(time: Time, f: string, zone: Timezone = local()): string
     {.raises: [TimeFormatParseError].} =