[rubygems/rubygems] Restore treating "--" as an unknown platform

Rather than crashing when parsing it.

https://github.com/rubygems/rubygems/commit/aa0064e4c7

lib/rubygems/platform.rb

@@ -90,7 +90,7 @@ class Gem::Platform
     when String then
       cpu, os = arch.sub(/-+$/, "").split("-", 2)
 
-      @cpu = if cpu.match?(/i\d86/)
+      @cpu = if cpu&.match?(/i\d86/)
         "x86"
       else
         cpu

test/rubygems/test_gem_platform.rb

@@ -162,6 +162,8 @@ class TestGemPlatform < Gem::TestCase
       "x86_64-dotnetx86" => ["x86_64", "dotnet", nil],
       "x86_64-dalvik0" => ["x86_64", "dalvik", "0"],
       "x86_64-dotnet1." => ["x86_64", "dotnet", "1"],
+
+      "--" => [nil, "unknown", nil],
     }
 
     test_cases.each do |arch, expected|