Offline export. When quote='#', the empty string is ""

**Bug Description**
Offline export. When quote='#', the empty string is ""

**Expected Behavior**
the empty string is ##

**Relation Case**
test_select_into_load_data.yaml id:0-1

**Steps to Reproduce**

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Look at the offline export functionality and the handling of quote characters, particularly when quote='#'. The test case 'test_select_into_load_data.yaml id:0-1' is a starting point. Examine the code that generates CSV output and how empty strings are quoted. Verify the expected behavior that an empty string should be represented as '##'.

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