NopLogger: Fix nil Clock panic

In #897, we added a `zap.Clock` option to control the source of time
but neglected to set this field on the logger constructed by
`zap.NewNop`. This has the effect of panicking the Nop logger with a nil
dereference.

Fix the nil dereference and add checks for the behavior of the Nop
logger.

Verified that these are the only instantiations of `Logger` in this
package:

```
$ rg '\bLogger\{' *.go
logger_test.go
67:                     for _, logger := range []*Logger{grandparent, parent, child} {

logger.go
71:     log := &Logger{
86:     return &Logger{
```

Refs GO-684

logger.go

@@ -87,6 +87,7 @@ func NewNop() *Logger {
 		core:        zapcore.NewNopCore(),
 		errorOutput: zapcore.AddSync(ioutil.Discard),
 		addStack:    zapcore.FatalLevel + 1,
+		clock:       zapcore.DefaultClock,
 	}
 }
 

logger_test.go

@@ -556,7 +556,6 @@ func TestLoggerCustomOnFatal(t *testing.T) {
 	for _, tt := range tests {
 		t.Run(tt.msg, func(t *testing.T) {
 			withLogger(t, InfoLevel, opts(OnFatal(tt.onFatal)), func(logger *Logger, logs *observer.ObservedLogs) {
-
 				var finished bool
 				recovered := make(chan interface{})
 				go func() {
@@ -580,6 +579,27 @@ func TestLoggerCustomOnFatal(t *testing.T) {
 	}
 }
 
+func TestNopLogger(t *testing.T) {
+	logger := NewNop()
+
+	t.Run("basic levels", func(t *testing.T) {
+		logger.Debug("foo", String("k", "v"))
+		logger.Info("bar", Int("x", 42))
+		logger.Warn("baz", Strings("ks", []string{"a", "b"}))
+		logger.Error("qux", Error(errors.New("great sadness")))
+	})
+
+	t.Run("DPanic", func(t *testing.T) {
+		logger.With(String("component", "whatever")).DPanic("stuff")
+	})
+
+	t.Run("Panic", func(t *testing.T) {
+		assert.Panics(t, func() {
+			logger.Panic("great sadness")
+		}, "Nop logger should still cause panics.")
+	})
+}
+
 func infoLog(logger *Logger, msg string, fields ...Field) {
 	logger.Info(msg, fields...)
 }