Abi: Add isNull() method

Change-Id: If8d6e5ace13857e4435b09e47d5d40dd2befdacf
Reviewed-on: http://codereview.qt.nokia.com/4279
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>

src/plugins/projectexplorer/abi.cpp

@@ -485,6 +485,15 @@ bool Abi::isValid() const
             && m_wordWidth != 0;
 }
 
+bool Abi::isNull() const
+{
+    return m_architecture == UnknownArchitecture
+            && m_os == UnknownOS
+            && m_osFlavor == UnknownFlavor
+            && m_binaryFormat == UnknownFormat
+            && m_wordWidth == 0;
+}
+
 QString Abi::toString(const Architecture &a)
 {
     switch (a) {