COMP: Do not force JsonCpp and ParameterSerializer DIR variable to empty string

This commit ensures project expected to find JsonCpp after finding
SlicerExecutionModel can do successfully even when SlicerExecutionModel
is built without ParameterSerializer support.

See Slicer/Slicer#5080

Author: jcfr

GenerateCLP/GenerateCLPConfig.cmake.in

@@ -12,9 +12,21 @@ set(SlicerExecutionModel_CMAKE_DIR "@SlicerExecutionModel_CMAKE_DIR@")
 set(TCLAP_DIR "@TCLAP_DIR@")
 set(ModuleDescriptionParser_DIR "@ModuleDescriptionParser_DIR@")
 set(ITK_DIR "@ITK_DIR_CONFIG@")
-set(JsonCpp_INCLUDE_DIR "@JsonCpp_INCLUDE_DIR@")
-set(JsonCpp_LIBRARY "@JsonCpp_LIBRARY@")
-set(ParameterSerializer_DIR "@ParameterSerializer_DIR@")
+
+set(GenerateCLP_JsonCpp_INCLUDE_DIR "@JsonCpp_INCLUDE_DIR@")
+if(NOT "${GenerateCLP_JsonCpp_INCLUDE_DIR}" STREQUAL "")
+  set(JsonCpp_INCLUDE_DIR ${GenerateCLP_JsonCpp_INCLUDE_DIR})
+endif()
+
+set(GenerateCLP_JsonCpp_LIBRARY "@JsonCpp_LIBRARY@")
+if(NOT "${GenerateCLP_JsonCpp_LIBRARY}" STREQUAL "")
+  set(JsonCpp_LIBRARY ${GenerateCLP_JsonCpp_LIBRARY})
+endif()
+
+set(GenerateCLP_ParameterSerializer_DIR "@ParameterSerializer_DIR@")
+if(NOT "${GenerateCLP_ParameterSerializer_DIR}" STREQUAL "")
+  set(ParameterSerializer_DIR ${GenerateCLP_ParameterSerializer_DIR})
+endif()
 
 find_program(GENERATECLP_EXE
   NAMES GenerateCLPLauncher GenerateCLP

SlicerExecutionModelConfig.cmake.in

@@ -146,9 +146,21 @@ endif()
 set(ModuleDescriptionParser_DIR ${SlicerExecutionModel_DIR}/ModuleDescriptionParser)
 set(GenerateCLP_DIR ${SlicerExecutionModel_DIR}/GenerateCLP)
 set(TCLAP_DIR ${SlicerExecutionModel_DIR}/tclap)
-set(JsonCpp_INCLUDE_DIR "@JsonCpp_INCLUDE_DIR@")
-set(JsonCpp_LIBRARY "@JsonCpp_LIBRARY@")
-set(ParameterSerializer_DIR "@ParameterSerializer_DIR@")
+
+set(SlicerExecutionModel_JsonCpp_INCLUDE_DIR "@JsonCpp_INCLUDE_DIR@")
+if(NOT "${SlicerExecutionModel_JsonCpp_INCLUDE_DIR}" STREQUAL "")
+  set(JsonCpp_INCLUDE_DIR ${SlicerExecutionModel_JsonCpp_INCLUDE_DIR})
+endif()
+
+set(SlicerExecutionModel_JsonCpp_LIBRARY "@JsonCpp_LIBRARY@")
+if(NOT "${SlicerExecutionModel_JsonCpp_LIBRARY}" STREQUAL "")
+  set(JsonCpp_LIBRARY ${SlicerExecutionModel_JsonCpp_LIBRARY})
+endif()
+
+set(SlicerExecutionModel_ParameterSerializer_DIR "@ParameterSerializer_DIR@")
+if(NOT "${SlicerExecutionModel_ParameterSerializer_DIR}" STREQUAL "")
+  set(ParameterSerializer_DIR ${SlicerExecutionModel_ParameterSerializer_DIR})
+endif()
 
 #
 # Find ModuleDescriptionParser, GenerateCLP and TCLAP so that the associated