[BUG] Cell.text throw error if cell is merged and isn't a master

[Siemienik]

🐛 Bug Report

When working on Siemienik/xlsx-renderer#43 founded issue with merged child cells

Steps To Reproduce

Conditions:

• Merged cell: A1:B1 with value hello world
• Cell A1 is master
• Cell B1 is a child

ws.getCell('A1').text // `hello world`
ws.getCell('B1').text // exception `TypeError: Cannot read property 'toString' of null` 

The expected behaviour:

ws.getCell('A1').text // `hello world`
ws.getCell('B1').text // `hello world`

Possible solution (optional, but very helpful):

Investigate and fix the behaviour of MergeValueType