src: fix accessing empty string

Author: zcbenz

src/node_dotenv.cc

@@ -156,7 +156,7 @@ void Dotenv::ParseContent(const std::string_view input) {
     key = trim_spaces(key);
 
     // If the value is not present (e.g. KEY=) set is to an empty string
-    if (content.front() == '\n') {
+    if (content.empty() || content.front() == '\n') {
       store_.insert_or_assign(std::string(key), "");
       continue;
     }