fix: welcome page user authentication check

[dpgaspar]

SUMMARY

Small "fix" for the public welcome page, by first asserting if g.user exists enables possible exceptions to be raised and handled accordingly on custom security managers (eg: JWT token exceptions).

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #20605 (181cba5) into master (309327d) will decrease coverage by 11.80%.
The diff coverage is 100.00%.

❗ Current head 181cba5 differs from pull request most recent head aab49aa. Consider uploading reports for the commit aab49aa to get more accurate results

@@             Coverage Diff             @@
##           master   #20605       +/-   ##
===========================================
- Coverage   66.79%   54.98%   -11.81%     
===========================================
  Files        1754     1754               
  Lines       65565    65565               
  Branches     6935     6935               
===========================================
- Hits        43793    36051     -7742     
- Misses      20017    27759     +7742     
  Partials     1755     1755               

Flag	Coverage Δ
hive	53.77% <100.00%> (ø)
mysql	?
postgres	?
presto	53.63% <100.00%> (ø)
python	58.37% <100.00%> (-24.49%)	⬇️
sqlite	?
unit	50.64% <0.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...t-ui-chart-controls/src/operators/pivotOperator.ts	100.00% <ø> (ø)
...controls/src/operators/timeComparePivotOperator.ts	100.00% <ø> (ø)
...t-controls/src/sections/echartsTimeSeriesQuery.tsx	50.00% <ø> (ø)
...et-ui-chart-controls/src/shared-controls/index.tsx	51.66% <100.00%> (ø)
superset/views/core.py	34.79% <100.00%> (-42.67%)	⬇️
superset/utils/dashboard_import_export.py	0.00% <0.00%> (-100.00%)	⬇️
superset/key_value/commands/upsert.py	0.00% <0.00%> (-89.59%)	⬇️
superset/key_value/commands/update.py	0.00% <0.00%> (-89.37%)	⬇️
superset/key_value/commands/delete.py	0.00% <0.00%> (-85.30%)	⬇️
superset/key_value/commands/delete_expired.py	0.00% <0.00%> (-80.77%)	⬇️
... and 281 more

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