CTHUB - Ensure Blank Values are Displayed Correctly in Metabase

**Title:**
Ensure Blank Values are Displayed Correctly in Metabase

**Description:**
Uploaded data with blank values are sometimes displayed as `temp_null` in Metabase. All blank values that are acceptable to upload as blanks should remain blank. This change should apply to all datasets uploaded to CTHUB and subsequently transferred to Metabase. It has been confirmed that the original data source contains blank values, not `temp_null`.

**Purpose and benefit to user:**
Users need to see accurate representations of their data. Displaying `temp_null` instead of blank values can lead to confusion and misinterpretation of data.

**Acceptance Criteria:**
- [ ] Given I am a data uploader, When I upload a dataset with acceptable blank values to CTHUB, then the blank values are displayed as blanks in Metabase.
- [ ] Given I am a data viewer, When I view a dataset in Metabase, then I do not see `temp_null` where there should be blank values.
- [ ] Given I am a data uploader, When I upload a dataset to CTHUB, then I can verify that all relevant blank fields are displayed correctly in Metabase.

**Development Checklist:**
- [ ] Investigate the data handling process in CTHUB to ensure it maintains blank values as blanks.
- [ ] Check that CTHUB correctly handles blank values and does not convert them to `temp_null` before transferring to Metabase.

**Notes:**
Here is a screenshot of the issue:

No files or tests are named. Start by tracing the CTHUB dataset upload and transfer path into Metabase, then reproduce the issue with a dataset containing acceptable blank values. Identify where blank values become `temp_null`; done means blanks remain blank in Metabase for uploaded datasets and the acceptance scenarios pass.

Written by the indexing model from the issue text.