Support empty interpolation (#55)

Author: jonatanklosko

grammar.js

@@ -295,7 +295,7 @@ module.exports = grammar({
 
     charlist: ($) => choice($._quoted_i_single, $._quoted_i_heredoc_single),
 
-    interpolation: ($) => seq("#{", $._expression, "}"),
+    interpolation: ($) => seq("#{", optional($._expression), "}"),
 
     escape_sequence: ($) =>
       token(

test/corpus/term/string.txt

@@ -81,6 +81,18 @@ nested interpolation
           (integer))))
     (quoted_content)))
 
+=====================================
+empty interpolation
+=====================================
+
+"#{}"
+
+---
+
+(source
+  (string
+    (interpolation)))
+
 =====================================
 escape sequence
 =====================================