Converting raster to vector by generating center lines?

GRASS GIS is able do this.

You need two steps:

1. r.thin

Thins non-null cells that denote linear features in a raster map layer.

2. r.to.vect

Converts a raster map into a vector map.

Now it worked for me as @RoVo suggested before. The problem were the no-data values in my input raster. I changed these not with gdal_translate as @AndreJ suggests (in comments of answer above), but with GRASS r.mapcalc. Here the modules that I used:

Replace all no data values:

processing.runalg('grass:r.mapcalculator',
                  {"amap": gPb_rlayer,
                   "formula": "if(A>0, 1, null())",
                   "GRASS_REGION_PARAMETER": "%f,%f,%f,%f" % (xmin, xmax, ymin, ymax),
                   "GRASS_REGION_CELLSIZE_PARAMETER": 1,
                   "outfile": mapcalc})

Thin raster layer to thin non-null cells:

processing.runalg('grass7:r.thin',
                  {"input": mapcalc,
                   "GRASS_REGION_PARAMETER": "%f,%f,%f,%f" % (xmin, xmax, ymin, ymax),
                   "output": thinned})

Raster to vector conversion:

processing.runalg('grass7:r.to.vect',
                  {"input": thinned,
                   "type": 0,
                   "GRASS_OUTPUT_TYPE_PARAMETER": 2,
                   "GRASS_REGION_PARAMETER": "%f,%f,%f,%f" % (xmin, xmax, ymin, ymax),
                   "output": centerlines})