fix generating pkg-config files with empty -I directives (fix #445)

cmake/em/pkg.pc.em

@@ -1,6 +1,6 @@
 Name: @(CATKIN_PACKAGE_PREFIX + PROJECT_NAME)
 Description: Description of @PROJECT_NAME
 Version: @PROJECT_VERSION
-Cflags: @(' '.join(['-I%s' % include for include in PROJECT_ABSOLUTE_INCLUDE_DIRS.split(';')]))
-Libs: -L@PROJECT_SPACE_DIR/lib @(' '.join([lib for lib in PKG_CONFIG_LIBRARIES_WITH_PREFIX.split(';')]))
-Requires: @(PROJECT_CATKIN_DEPENDS.replace(';', ' '))
+Cflags: @(' '.join(['-I%s' % include for include in PROJECT_ABSOLUTE_INCLUDE_DIRS]))
+Libs: -L@PROJECT_SPACE_DIR/lib @(' '.join(PKG_CONFIG_LIBRARIES_WITH_PREFIX))
+Requires: @(PROJECT_CATKIN_DEPENDS)

cmake/templates/pkg.context.pc.in

@@ -1,7 +1,8 @@
-CATKIN_PACKAGE_PREFIX="@CATKIN_PACKAGE_PREFIX@"
-PROJECT_ABSOLUTE_INCLUDE_DIRS="@PROJECT_ABSOLUTE_INCLUDE_DIRS@"
-PROJECT_CATKIN_DEPENDS="@_PROJECT_CATKIN_DEPENDS@"
-PKG_CONFIG_LIBRARIES_WITH_PREFIX="@PKG_CONFIG_LIBRARIES_WITH_PREFIX@"
-PROJECT_NAME="@PROJECT_NAME@"
-PROJECT_SPACE_DIR="@PROJECT_SPACE_DIR@"
-PROJECT_VERSION="@PROJECT_VERSION@"
+# generated from catkin/cmake/template/pkg.context.pc.in
+CATKIN_PACKAGE_PREFIX = "@CATKIN_PACKAGE_PREFIX@"
+PROJECT_ABSOLUTE_INCLUDE_DIRS = "@PROJECT_ABSOLUTE_INCLUDE_DIRS@".split(';') if "@PROJECT_ABSOLUTE_INCLUDE_DIRS@" != "" else []
+PROJECT_CATKIN_DEPENDS = "@_PROJECT_CATKIN_DEPENDS@".replace(';', ' ')
+PKG_CONFIG_LIBRARIES_WITH_PREFIX = "@PKG_CONFIG_LIBRARIES_WITH_PREFIX@".split(';') if "@PKG_CONFIG_LIBRARIES_WITH_PREFIX@" != "" else []
+PROJECT_NAME = "@PROJECT_NAME@"
+PROJECT_SPACE_DIR = "@PROJECT_SPACE_DIR@"
+PROJECT_VERSION = "@PROJECT_VERSION@"