fix(empty dashboards): Allow downloading a screenshot of an empty dashboard

[msyavuz]

SUMMARY

Check if a dashboard contains charts first, if not don't wait for them to load etc. This fixes the bug where if the dashboard is empty screenshot fails.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

1. Create an empty dashboard
2. Download png/pdf screenshot of the empty dashboard

ADDITIONAL INFORMATION

• Has associated issue: Fixes Dashboard Screenshot API Endpooint Gives and Error Message when a Dashboard is Empty or has no Charts  #30736
• 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

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 83.91%. Comparing base (76d897e) to head (38032e5).
Report is 966 commits behind head on master.

Files with missing lines	Patch %	Lines
superset/utils/webdriver.py	0.00%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #30767       +/-   ##
===========================================
+ Coverage   60.48%   83.91%   +23.43%     
===========================================
  Files        1931      536     -1395     
  Lines       76236    38921    -37315     
  Branches     8568        0     -8568     
===========================================
- Hits        46114    32662    -13452     
+ Misses      28017     6259    -21758     
+ Partials     2105        0     -2105     

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[geido]

I did some text enhancements for comments and errors but the general approach looks fine to me. It would be nice to have a better way to differentiate between an empty dashboard (that will never load any charts) and a dashboard that is waiting for chart containers to draw, but I guess this should do fine for now. I tested it locally too. Thanks!