Boost: Treat empty CXX, AR, RANLIB env as undefined

Fixes Issue #5569

A bug in cmake-gui would have it sometimes set CXX to
empty instead of leaving it undefined.  CXX detection
in the boost recipe would use the blank value and fail
to build.  Now we treat empty the same as undefined
and move on to e.g. xcrun detection for these properties.

recipes/boost/all/conanfile.py

@@ -1044,23 +1044,23 @@ def _build_cross_flags(self):
 
     @property
     def _ar(self):
-        if "AR" in os.environ:
+        if os.environ.get("AR"):
             return os.environ["AR"]
         if tools.is_apple_os(self.settings.os) and self.settings.compiler == "apple-clang":
             return tools.XCRun(self.settings).ar
         return None
 
     @property
     def _ranlib(self):
-        if "RANLIB" in os.environ:
+        if os.environ.get("RANLIB"):
             return os.environ["RANLIB"]
         if tools.is_apple_os(self.settings.os) and self.settings.compiler == "apple-clang":
             return tools.XCRun(self.settings).ranlib
         return None
 
     @property
     def _cxx(self):
-        if "CXX" in os.environ:
+        if os.environ.get("CXX"):
             return os.environ["CXX"]
         if tools.is_apple_os(self.settings.os) and self.settings.compiler == "apple-clang":
             return tools.XCRun(self.settings).cxx