fixes panic appears when tailer is nil

[k-kinzal]

A go panic occurs when the log file specification is empty.
The problem is that Collect works when the tailer is nil.

$ /bin/postfix_exporter --postfix.logfile_path=

$ kc logs -c exporter postfix-daemon-kr7mp
2020/04/22 03:09:38 Reading log events from 
2020/04/22 03:09:38 Listening on :9154
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x841538]

goroutine 10 [running]:
main.(*PostfixExporter).CollectLogfileFromFile(0xc0000c4dc0, 0xa02c60, 0xc00002d080)
	/go/src/github.com/kumina/postfix_exporter/postfix_exporter.go:433 +0xe8
main.(*PostfixExporter).StartMetricCollection(0xc0000c4dc0, 0xa02c60, 0xc00002d080)
	/go/src/github.com/kumina/postfix_exporter/postfix_exporter.go:671 +0x112
created by main.main
	/go/src/github.com/kumina/postfix_exporter/main.go:69 +0xedb

[BartVerc]

Thanks for the PR!