fix: extra separator

[lamweili]

Fixes #64

Related to log4js-node/log4js-node#1039

const log4js = require('log4js');
log4js.configure({
  appenders: {
    test1: { 
      type: 'dateFile', 
      filename: 'test1.log', 
      pattern: 'yyyy-MM-dd', 
      alwaysIncludePattern: true, 
      keepFileExt: true
    },
    test2: { 
      type: 'dateFile', 
      filename: 'test2.log', 
      pattern: 'yyyy-MM-dd',
      fileNameSep: '_',
      alwaysIncludePattern: true, 
      keepFileExt: true
    },
    test3: { 
      type: 'dateFile', 
      filename: 'test3.log', 
      pattern: '.yyyy-MM-dd', 
      alwaysIncludePattern: true, 
      keepFileExt: true
    },
    test4: { 
      type: 'dateFile', 
      filename: 'test4.log', 
      pattern: '-yyyy-MM-dd', 
      alwaysIncludePattern: true, 
      keepFileExt: true
    },
    test5: { 
      type: 'dateFile', 
      filename: 'test5.log', 
      pattern: '.-yyyy-MM-dd', 
      alwaysIncludePattern: true, 
      keepFileExt: true
    },
  },
  categories: {
    default: { appenders: [ 'test1', 'test2', 'test3', 'test4', 'test5' ], level: 'debug' }
  }
});

Pattern	Separator	Expected	Actual	Test
yyyy-MM-dd	. (default)	test1.2022-01-07.log	test1.2022-01-07.log	✔️
yyyy-MM-dd	_	test2_2022-01-07.log	test2_2022-01-07.log	✔️
.yyyy-MM-dd	. (default)	test3..2022-01-07.log	test3..2022-01-07.log	✔️
-yyyy-MM-dd	. (default)	test4.-2022-01-07.log	test4.-2022-01-07.log	✔️
.-yyyy-MM-dd	. (default)	test5..-2022-01-07.log	test5..-2022-01-07.log	✔️