fix(django): Fix errors when instrumenting Django cache

[BYK]

I was testing Spotlight with Sentry and realized things started to get slow and crashy. It looks like sometimes args is just an empty array on cache's _instruments_call causing lots of exceptions being thrown. This patch fixes that with explicit length checks and also adds a note for the missing instrumentation for get_or_set method. This might be related to #2122 and #3300.

[antonpirker]

Nice, thanks!

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.92%. Comparing base (5891717) to head (5ea33a4).
Report is 1 commits behind head on master.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
sentry_sdk/integrations/django/caching.py	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #3855       +/-   ##
===========================================
+ Coverage   66.07%   79.92%   +13.84%     
===========================================
  Files         137      137               
  Lines       15403    15404        +1     
  Branches     2618     2620        +2     
===========================================
+ Hits        10177    12311     +2134     
+ Misses       4346     2221     -2125     
+ Partials      880      872        -8     

Files with missing lines	Coverage Δ
sentry_sdk/integrations/django/caching.py	92.92% <66.66%> (-0.95%)	⬇️

... and 40 files with indirect coverage changes