[yaml2obj] Do not write the string table if there is no string entry.

[Esme]

Summary

This is a minor patch from James's comments in D105522.
I will add a test for this after "llvm-readobj --string-table" supports printing the size of string tables.

[Higuoxing]

LGTM, thanks!

[jhenderson]

LGTM. This makes sense to me.

llvm/lib/ObjectYAML/XCOFFEmitter.cpp

bool XCOFFWriter::writeXCOFF() {
   writeFileHeader();
   if (!Obj.Sections.empty()) {
     writeSectionHeader();
     if (!writeSectionData())
       return false;
     if (!writeRelocations())
       return false;
   }
-  if (!Obj.Symbols.empty()) {
-    if (!writeSymbols())
-      return false;
-    // Write the string table.
+  if (!Obj.Symbols.empty() && !writeSymbols())
+  if (Strings.getSize() > 4)
     Strings.write(W.OS);
-  }
   return true;
 }
 
 } // end anonymous namespace
 
 namespace llvm {
 namespace yaml {
 
 bool yaml2xcoff(XCOFFYAML::Object &Doc, raw_ostream &Out, ErrorHandler EH) {
   XCOFFWriter Writer(Doc, Out, EH);
   return Writer.writeXCOFF();
 }
 
 } // namespace yaml
 } // namespace llvm