use empty method to check for emptyness

Source/cmAddCompileOptionsCommand.cxx

Source/cmAddCustomTargetCommand.cxx

Source/cmAddDefinitionsCommand.cxx

Source/cmAddLibraryCommand.cxx

Source/cmAddSubDirectoryCommand.cxx

Source/cmBuildCommand.cxx

Source/cmBuildNameCommand.cxx

Source/cmCMakePolicyCommand.cxx

Source/cmConditionEvaluator.cxx

Source/cmDefinePropertyCommand.cxx

Source/cmEnableLanguageCommand.cxx

Source/cmExecProgramCommand.cxx

Source/cmExecuteProcessCommand.cxx

Source/cmExportBuildAndroidMKGenerator.cxx

Source/cmExportInstallAndroidMKGenerator.cxx

Source/cmExportLibraryDependenciesCommand.cxx

Source/cmFindPackageCommand.cxx

Source/cmForEachCommand.cxx

Source/cmFunctionCommand.cxx

Source/cmIncludeCommand.cxx