QiangCai opened a new pull request #3898:
URL: https://github.com/apache/carbondata/pull/3898
### Why is this PR needed?
1. column comment is an empty string by default when adding a column
2. there are too many redundancy codes
### What changes were proposed in this PR?
1. change default column comment to null
2. re-factory code
### Does this PR introduce any user interface change?
- No
### Is any new testcase added?
- Yes
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