ENH: More robust property lookup

This teaches cmMakefile::GetProperty and cmake::GetProperty methods to
return NULL when the property name is NULL, making them more robust and
consistent with the behavior of cmTarget::GetProperty.

Source/cmMakefile.cxx

@@ -3162,6 +3162,10 @@ const char *cmMakefile::GetProperty(const char* prop)
 const char *cmMakefile::GetProperty(const char* prop,
                                     cmProperty::ScopeType scope)
 {
+  if(!prop)
+    {
+    return 0;
+    }
   // watch for specific properties
   static std::string output;
   output = "";

Source/cmake.cxx

@@ -3606,6 +3606,10 @@ const char *cmake::GetProperty(const char* prop)
 
 const char *cmake::GetProperty(const char* prop, cmProperty::ScopeType scope)
 {
+  if(!prop)
+    {
+    return 0;
+    }
   bool chain = false;
 
   // watch for special properties