Handle timestamp 0 in prettifyTime

When trying to format a log object like the following:

```json
{
  "level": 30,
  "time": 0,
  "name": "foobar",
  "msg": "I like cake",
  "v": 1
}
```

pino-pretty previously tried to format the timestamp `undefined` instead of
taking the `0` time. This commit looks not at the truthiness of the value, but
its existence

lib/utils.js

@@ -326,9 +326,19 @@ function prettifyObject ({
  * string.
  */
 function prettifyTime ({ log, timestampKey = TIMESTAMP_KEY, translateFormat = undefined }) {
-  if (timestampKey in log === false && 'timestamp' in log === false) return undefined
+  let time = null
+
+  if (timestampKey in log) {
+    time = log[timestampKey]
+  }
+  else if ('timestamp' in log) {
+    time = log.timestamp
+  }
+
+  if (time === null) return undefined
   if (translateFormat) {
-    return '[' + formatTime(log[timestampKey] || log.timestamp, translateFormat) + ']'
+    return '[' + formatTime(time, translateFormat) + ']'
   }
-  return `[${log[timestampKey] || log.timestamp}]`
+
+  return `[${log}]`
 }