fix: skip None when sorting unique values for range condition

[iabaako]

Pull Request Summary 🚀

What does this PR do? 📝

Fixes a crash in the "remove rows" prep step's range condition when a
selected column contains a mix of floats and nulls.
_get_unique_values_from_columns now filters out None before calling
sorted().

Fixes #281

Why is this change needed? 🤔

Python can't compare float and NoneType, so sorted() raised
TypeError: '<' not supported between instances of 'float' and 'NoneType'
whenever a selected range-condition column had missing values, crashing the
app on every rerun of that prep step.

How was this implemented? 🛠️

In src/datasure/views/prep_view.py, _get_unique_values_from_columns
(line 504) now sorts (v for v in value_options if v is not None) instead
of sorting the raw list. Nulls aren't meaningful bounds for a range
condition, so dropping them before sort is correct and minimal.

How to test or reproduce ? 🧪

1. Start the app (uv run datasure) and open a project's prep view
2. Add a "remove rows" prep step
3. Choose a range condition and select a column containing both numeric
   values and missing/null values
4. Before the fix: app crashes with the TypeError above
5. After the fix: min/max selectboxes populate with the non-null values

Screenshots (if applicable) 📷

N/A (backend logic fix, no UI change beyond no longer crashing)

Checklist ✅

• I have run and tested my changes locally
• I have limit this PR to less than 1000 lines of code change (if not, explain why)
• I have updated/added tests to cover my changes (if applicable)
• I have updated/added requirements to cover my changes (if applicable)
• I have run linting and formatting on any code changes (if applicable)
• I have updated the documentation (README, etc.) accordingly
• I have reviewed and resolved any merge conflict

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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