Treat an empty source map string as missing

Summary:
Currently, an empty string will be treated as a source map, while nullptr will
treat it as missing. For easier JS interop, treat these the same.

Reviewed By: ArchDev

Differential Revision: D21621982

fbshipit-source-id: c408b3889ce368ce4bad5b4529deca8c1eeb9c40

tools/emhermesc/emhermesc.cpp

@@ -121,7 +121,7 @@ extern "C" CompileResult *hermesCompileToBytecode(
     return compileRes.release();
   }
 
-  if (sourceMapData != nullptr) {
+  if (sourceMapData != nullptr && sourceMapData[0] != '\0') {
     if (sourceMapData[sourceMapSize - 1] != 0) {
       compileRes->error_ = "Input sourcemap must be zero-terminated";
       return compileRes.release();