[svn ci] (r11911) .None PMC now returns empty string instead of "None" for get_string

[jerry gay]

previously, the get_string method of the .None PMC returned "None".
it's been modified to return the somewhat more sane "", or empty
string. this may affect code using .Hash PMCs, as the get_string
method of the .None PMC is called when the value of a non-existant
hash key is requested in string context. for example:

    P0 = new .Hash
    S0 = P0['abc']
    eq S0, '', OK
    print 'not '
OK: print 'ok'
    end

## prints 'ok'

all affected tests and library files have been modified.
~jerry