Fix issue with indented partials (#19)

* Fix issue with indented partials

* Add test

* Add empty transform for string

Sources/HummingbirdMustache/Template+Render.swift

@@ -27,14 +27,16 @@ extension HBMustacheTemplate {
 
         if let indentation = context.indentation, indentation != "" {
             for token in tokens {
-                if string.last == "\n" {
+                let renderedString = self.renderToken(token, context: &context)
+                if renderedString != "", string.last == "\n" {
                     string += indentation
                 }
-                string += self.renderToken(token, context: &context)
+                string += renderedString
             }
         } else {
             for token in tokens {
-                string += self.renderToken(token, context: &context)
+                let result = self.renderToken(token, context: &context)
+                string += result
             }
         }
         return string

Sources/HummingbirdMustache/Transform.swift

@@ -39,6 +39,8 @@ public extension StringProtocol {
     /// - Returns: Result
     func transform(_ name: String) -> Any? {
         switch name {
+        case "empty":
+            return isEmpty
         case "capitalized":
             return capitalized
         case "lowercased":