fix: correcting S3 nil cipher dereference in filer init (#7952)

Resolves the following error reported in #7949:
```
I0103 21:38:30.230662 s3.go:275 Starting S3 API Server with standard IAM
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x38ca961]

goroutine 102 [running]:
github.com/seaweedfs/seaweedfs/weed/command.(*S3Options).startS3Server(0x7caf840)
    /go/src/github.com/seaweedfs/seaweedfs/weed/command/s3.go:295 +0x741
github.com/seaweedfs/seaweedfs/weed/command.runFiler.func1(...)
    /go/src/github.com/seaweedfs/seaweedfs/weed/command/filer.go:244
created by github.com/seaweedfs/seaweedfs/weed/command.runFiler in goroutine 1
    /go/src/github.com/seaweedfs/seaweedfs/weed/command/filer.go:242 +0x353
```

weed/command/filer.go

@@ -137,6 +137,7 @@ func init() {
 	filerS3Options.concurrentUploadLimitMB = cmdFiler.Flag.Int("s3.concurrentUploadLimitMB", 0, "limit total concurrent upload size for S3, 0 means unlimited")
 	filerS3Options.concurrentFileUploadLimit = cmdFiler.Flag.Int("s3.concurrentFileUploadLimit", 0, "limit number of concurrent file uploads for S3, 0 means unlimited")
 	filerS3Options.enableIam = cmdFiler.Flag.Bool("s3.iam", true, "enable embedded IAM API on the same S3 port")
+	filerS3Options.cipher = cmdFiler.Flag.Bool("s3.encryptVolumeData", false, "encrypt data on volume servers for S3 uploads")
 
 	// start webdav on filer
 	filerStartWebDav = cmdFiler.Flag.Bool("webdav", false, "whether to start webdav gateway")