-xr switch generates unnecessary commas on imports

Run the provided a.py, b.py, and c.py example with transcrypt c.py -xr --nomin and inspect the generated imports. Trace the -xr import-generation path that combines imports from a and b. Done means the generated c.js contains import {test2} from './a.js'; without a leading comma.

Example to reproduce:
a.py:

def test1():
    pass

def test2():
    pass

b.py:

from a import test1

c.py:

from a import test1, test2
from b import *

Running ```transcrypt c.py -xr --nomin`` generates the following c.js:

import {AssertionError, AttributeError, BaseException, DeprecationWarning, Exception, IndexError, IterableError, KeyError, NotImplementedError, RuntimeWarning, StopIteration, UserWarning, ValueError, Warning, __JsIterator__, __PyIterator__, __Terminal__, __add__, __and__, __call__, __class__, __envir__, __eq__, __floordiv__, __ge__, __get__, __getcm__, __getitem__, __getslice__, __getsm__, __gt__, __i__, __iadd__, __iand__, __idiv__, __ijsmod__, __ilshift__, __imatmul__, __imod__, __imul__, __in__, __init__, __ior__, __ipow__, __irshift__, __isub__, __ixor__, __jsUsePyNext__, __jsmod__, __k__, __kwargtrans__, __le__, __lshift__, __lt__, __matmul__, __mergefields__, __mergekwargtrans__, __mod__, __mul__, __ne__, __neg__, __nest__, __or__, __pow__, __pragma__, __proxy__, __pyUseJsNext__, __rshift__, __setitem__, __setproperty__, __setslice__, __sort__, __specialattrib__, __sub__, __super__, __t__, __terminal__, __truediv__, __withblock__, __xor__, abs, all, any, assert, bool, bytearray, bytes, callable, chr, copy, deepcopy, delattr, dict, dir, divmod, enumerate, filter, float, getattr, hasattr, id, input, int, isinstance, issubclass, len, list, map, max, min, object, ord, pow, print, property, py_TypeError, py_iter, py_metatype, py_next, py_reversed, py_typeof, range, repr, round, set, setattr, sorted, str, sum, tuple, zip} from './org.transcrypt.__runtime__.js';
import {test1} from './b.js';
import {, test2} from './a.js';
export {test1, test2};
var __name__ = '__main__';

The bugged line is
import {, test2} from './a.js';