Fix NSAppTransportSecurity being overwritten during pod install (#41875)

Summary:
This PR fixes https://github.com/facebook/react-native/issues/41874.

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[iOS] [Fixed] - Fix NSAppTransportSecurity being overwritten during pod install

Pull Request resolved: https://github.com/facebook/react-native/pull/41875

Test Plan:
1. Delete the whole `NSAppTransportSecurity` in Info.plist and run `pod install`: `NSAllowsArbitraryLoads` and `NSAllowsLocalNetworking` are added as expected.
2. Modify `NSAppTransportSecurity` to only contain `NSExceptionDomains` and run `pod install`: `NSAllowsArbitraryLoads` and `NSAllowsLocalNetworking` are added, and `NSExceptionDomains` is still there.
3. Run `pod install` again: nothing changes.

Reviewed By: christophpurrer

Differential Revision: D52032400

Pulled By: dmytrorykun

fbshipit-source-id: 48cf29809c283af80613ffbf1ac0dc663a0a2fb5

packages/react-native/scripts/cocoapods/utils.rb

@@ -541,7 +541,8 @@ class ReactNativePodsUtils
                     "NSAppTransportSecurity" => ats_configs
                 }
             else
-                plist["NSAppTransportSecurity"] = ats_configs
+                plist["NSAppTransportSecurity"] ||= {}
+                plist["NSAppTransportSecurity"] = plist["NSAppTransportSecurity"].merge(ats_configs)
             end
             Xcodeproj::Plist.write_to_path(plist, fullPlistPath)
         end