no blank line if no LOC

Makefile

@@ -17,7 +17,8 @@ makefiles = \
   misc/upstart/local.mk \
   doc/manual/local.mk \
   tests/local.mk \
-  tests/plugins/local.mk
+  tests/plugins/local.mk \
+  src/error-demo/local.mk
 
 -include Makefile.config
 

src/error-demo/error-demo.cc

@@ -1,5 +1,6 @@
 #include "logging.hh"
 #include "nixexpr.hh"
+#include "util.hh"
 
 #include <iostream>
 #include <optional>

src/libutil/error.cc

@@ -182,7 +182,7 @@ std::ostream& operator<<(std::ostream &out, const ErrorInfo &einfo)
     }
 
     // lines of code.
-    if (einfo.nixCode.has_value()) {
+    if (einfo.nixCode.has_value() && einfo.nixCode->errLineOfCode.has_value()) {
         printCodeLines(out, prefix, *einfo.nixCode);
         out << prefix << std::endl;
     }