[ruby/prism] Fix parser translator ast for empty regex

In that specific case, no string node is emitted

ruby/prism@1166db13dd

Author: Earlopain

lib/prism/translation/parser/compiler.rb

@@ -1512,7 +1512,9 @@ def visit_redo_node(node)
         # ^^^^^
         def visit_regular_expression_node(node)
           parts =
-            if node.content.include?("\n")
+            if node.content == ""
+              []
+            elsif node.content.include?("\n")
               string_nodes_from_line_continuations(node, node.content_loc.start_offset, node.opening)
             else
               [builder.string_internal(token(node.content_loc))]

test/prism/fixtures/regex.txt

@@ -54,3 +54,5 @@ d\\\
 e\\
 f\
 /
+
+//