Fix #1129 Setting schema-less URL for proxy URLs results in NullPointerException

[seratch]

This pull request resolves #1129

Category (place an x in each of the [ ])

• bolt (Bolt for Java)
• bolt-{sub modules} (Bolt for Java - optional modules)
• slack-api-client (Slack API Clients)
• slack-api-model (Slack API Data Models)
• slack-api-*-kotlin-extension (Kotlin Extensions for Slack API Clients)
• slack-app-backend (The primitive layer of Bolt for Java)

Requirements

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[codecov]

Codecov Report

Merging #1133 (7592445) into main (dc788dc) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               main    #1133   +/-   ##
=========================================
  Coverage     75.75%   75.75%           
- Complexity     3812     3816    +4     
=========================================
  Files           414      414           
  Lines         11866    11867    +1     
  Branches       1174     1178    +4     
=========================================
+ Hits           8989     8990    +1     
+ Misses         2172     2171    -1     
- Partials        705      706    +1     

Impacted Files	Coverage Δ
...ain/java/com/slack/api/util/http/ProxyUrlUtil.java	86.66% <100.00%> (+0.45%)	⬆️

... and 2 files with indirect coverage changes

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