Ensure that conversions from/to 8bit and 16bit never return null string.

Source/wtf/text/WTFString.cpp

Index: Source/wtf/text/WTFString.cpp
diff --git a/Source/wtf/text/WTFString.cpp b/Source/wtf/text/WTFString.cpp
index 1ae107ea6aebee860a7c1c814357bcd5ed5fd4ef..d72969f19a6a82d4f4f9e513510ec2b01ad2749f 100644
--- a/Source/wtf/text/WTFString.cpp
+++ b/Source/wtf/text/WTFString.cpp
@@ -874,7 +874,7 @@ CString String::utf8(UTF8ConversionMode mode) const
 String String::make8BitFrom16BitSource(const UChar* source, size_t length)
 {
     if (!length)
-        return String();
+        return emptyString();
 
     LChar* destination;
     String result = String::createUninitialized(length, destination);
@@ -887,7 +887,7 @@ String String::make8BitFrom16BitSource(const UChar* source, size_t length)
 String String::make16BitFrom8BitSource(const LChar* source, size_t length)
 {
     if (!length)
-        return String();
+        return emptyString();
 
     UChar* destination;
     String result = String::createUninitialized(length, destination);