make attribute CFLAGS and LDFLAGS optional and add default empty value

Actually CFLAGS and LDFLAGS are not required and can be empty.
Without default empty value if target platform in project settings was
set to "Linux" CFLAGS and LDFLAGS was initialized with NoneType.
The result was broken build and it wasn't to save/load project with
such settings.

Traceback (most recent call last):
File "/home/developer/WorkData/PLC/beremiz/beremiz/ProjectController.py", line 956, in _Build
if not builder.build() :
File "/home/developer/WorkData/PLC/beremiz/beremiz/targets/toolchain_gcc.py", line 96, in build
Builder_CFLAGS = ' '.join(self.getBuilderCFLAGS())
TypeError: sequence item 0: expected string, NoneType found

libraries = [
    ('Native', 'NativeLib.NativeLibrary'),
    ('Python', 'py_ext.PythonLibrary'),
    ('SVGUI', 'svgui.SVGUILibrary')]

catalog = [
    ('canfestival', _('CANopen support'), _('Map located variables over CANopen'), 'canfestival.canfestival.RootClass'),
    ('c_ext', _('C extension'), _('Add C code accessing located variables synchronously'), 'c_ext.CFile'),
    ('py_ext', _('Python file'), _('Add Python code executed asynchronously'), 'py_ext.PythonFile'),
    ('wxglade_hmi', _('WxGlade GUI'), _('Add a simple WxGlade based GUI.'), 'wxglade_hmi.WxGladeHMI'),
    ('svgui', _('SVGUI'), _('Experimental web based HMI'), 'svgui.SVGUI')]

file_editors = []