Fix extra begin within dsym edge case

Extracted from #392

```bash
Original-Source:
{ "#{}": 42 }
Generated-Source:
{ :"#{()}" => 42 }
Original-Node:
(hash
  (pair
    (dsym
      (begin))
    (int 42)))
Generated-Node:
(hash
  (pair
    (dsym
      (begin
        (begin)))
    (int 42)))
Node-Diff:
@@ -1,5 +1,6 @@
 (hash
   (pair
     (dsym
-      (begin))
+      (begin
+        (begin)))
     (int 42)))
Error: (string)
```

Author: viralpraxis

lib/unparser/emitter/dsym.rb

@@ -33,7 +33,7 @@ def emit_str_child(value)
 
       def emit_begin_child(component)
         write('#{')
-        visit(unwrap_single_begin(component))
+        visit(unwrap_single_begin(component)) if component.children.any?
         write('}')
       end
     end # DSym

test/corpus/semantic/literal.rb

@@ -12,3 +12,4 @@
 10.2e10000000000
 -10.2e10000000000
 w(foo bar)
+{ "#{}": {} }