common: never pass None to replace_config_vars

common: never pass None to replace_config_vars

This fixes fdroid build com.umang.dashnotifier:18:

	ERROR: Could not build app com.umang.dashnotifier due to unknown error: Traceback (most recent call last):
	  File "/home/mvdan/git/fsr/fdroidserver/build.py", line 1121, in main
	    options.onserver, options.refresh):
	  File "/home/mvdan/git/fsr/fdroidserver/build.py", line 950, in trybuild
	    build_local(app, build, vcs, build_dir, output_dir, srclib_dir, extlib_dir, tmp_dir, force, onserver, refresh)
	  File "/home/mvdan/git/fsr/fdroidserver/build.py", line 480, in build_local
	    extlib_dir, onserver, refresh)
	  File "/home/mvdan/git/fsr/fdroidserver/common.py", line 1335, in prepare_source
	    srclibpaths.append(getsrclib(lib, srclib_dir, build, preponly=onserver, refresh=refresh))
	  File "/home/mvdan/git/fsr/fdroidserver/common.py", line 1258, in getsrclib
	    cmd = replace_config_vars(srclib["Prepare"], None)
	  File "/home/mvdan/git/fsr/fdroidserver/common.py", line 1822, in replace_config_vars
	    cmd = cmd.replace('$$NDK$$', build.ndk_path())
	AttributeError: 'NoneType' object has no attribute 'ndk_path'