Simplify begin environment parsing

Agent-Logs-Url: https://github.com/wesleyel/swiftui-math/sessions/56436444-e15b-4dd0-8a70-c87df1e3dc4e

Co-authored-by: wesleyel <48174882+wesleyel@users.noreply.github.com>

Sources/SwiftUIMath/Internal/Syntax/Parser.swift

@@ -805,11 +805,7 @@ extension Math {
 
         return table
       } else if command == "begin" {
-        let env = self.readEnvironment()
-        if env == nil {
-          return nil
-        }
-        guard let env else {
+        guard let env = self.readEnvironment() else {
           return nil
         }
         guard let environmentOptions = self.readEnvironmentOptions(for: env) else {