fix cmake error on mac

Author: Barthelemy

cmake/ReadoutCardDependencies.cmake

@@ -89,15 +89,21 @@ o2_define_bucket(
 )
 
 # Build the variable name for the path to boost python
-string(TOUPPER ${boost_python_component} upper)
-set(temp "Boost_${upper}_LIBRARY")
+set(boost_python_dep "")
+if(boost_python_component)
+    if (boost_python_component NOT STREQUAL "")
+        string(TOUPPER ${boost_python_component} upper)
+        set(temp "Boost_${upper}_LIBRARY")
+        set(boost_python_dep ${${temp}})
+    endif()
+endif()
 
 o2_define_bucket(
   NAME
   o2_readoutcard_python
 
   DEPENDENCIES
-  ${${temp}}
+  ${boost_python_dep}
   ${PYTHON_LIBRARIES}
 
   SYSTEMINCLUDE_DIRECTORIES