Fix display of empty string constant (#3207).

src/V3Number.h

@@ -233,6 +233,7 @@ public:
     V3Number(String, AstNode* nodep, const string& value) {
         init(nodep, 0);
         setString(value);
+        m_fromString = true;
     }
     class Null {};
     V3Number(Null, AstNode* nodep) {

src/verilog.y

@@ -5006,7 +5006,15 @@ str<strp>:			// yaSTRING but with \{escapes} need decoded
 
 strAsInt<nodep>:
 		yaSTRING
-			{ $$ = new AstConst{$<fl>1, AstConst::VerilogStringLiteral(), GRAMMARP->deQuote($<fl>1, *$1)}; }
+			{ if ($1->empty()) {
+			      // else "" is not representable as number as is width 0
+                              // TODO all strings should be represented this way
+			      // until V3Width converts as/if needed to a numerical constant
+			      $$ = new AstConst{$<fl>1, AstConst::String{}, GRAMMARP->deQuote($<fl>1, *$1)};
+			  } else {
+			      $$ = new AstConst{$<fl>1, AstConst::VerilogStringLiteral(), GRAMMARP->deQuote($<fl>1, *$1)};
+			  }
+			}
 	;
 
 strAsIntIgnore<nodep>:		// strAsInt, but never matches for when expr shouldn't parse strings