parser: Remove empty multiline string parts earlier

src/libexpr/parser-state.hh

@@ -291,7 +291,10 @@ inline Expr * ParserState::stripIndentation(const PosIdx pos,
                 s2 = std::string(s2, 0, p + 1);
         }
 
-        es2->emplace_back(i->first, new ExprString(std::move(s2)));
+        // Ignore empty strings for a minor optimisation and AST simplification
+        if (s2 != "") {
+            es2->emplace_back(i->first, new ExprString(std::move(s2)));
+        }
     };
     for (; i != es.end(); ++i, --n) {
         std::visit(overloaded { trimExpr, trimString }, i->second);

tests/functional/lang/parse-okay-ind-string.exp

@@ -0,0 +1 @@
+(let string = "str"; in [ (/some/path) ((/some/path)) ((/some/path)) ((/some/path + "\n    end")) (string) ((string)) ((string)) ((string + "\n    end")) ])

tests/functional/lang/parse-okay-ind-string.nix

@@ -0,0 +1,23 @@
+let
+  string = "str";
+in [
+  /some/path
+
+  ''${/some/path}''
+
+  ''
+    ${/some/path}''
+
+  ''${/some/path}
+    end''
+
+  string
+
+  ''${string}''
+
+  ''
+    ${string}''
+
+  ''${string}
+    end''
+]