fix: Do not overwrite user ip if set manually in ASP.NET sdk

[alexsohn1126]

Fixes #3337

Problem

We were overwriting Event's user IP address in ASP.NET if it was set manually.

Solution

We only populate an event's user ip if it was not set before.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 73.43%. Comparing base (4a2bf5f) to head (a482e19).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
....AspNet/Internal/SystemWebRequestEventProcessor.cs	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4513      +/-   ##
==========================================
+ Coverage   73.38%   73.43%   +0.05%     
==========================================
  Files         479      479              
  Lines       17506    17506              
  Branches     3479     3480       +1     
==========================================
+ Hits        12846    12856      +10     
+ Misses       3780     3772       -8     
+ Partials      880      878       -2     

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[alexsohn1126]

bugbot review

[cursor]

✅ Bugbot reviewed your changes and found no bugs!

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[jamescrosswell]

Nice - thanks @alexsohn1126 !