ReFrame crashes if `sourcesdir` is set to `None` in class body

[vkarak]

The following test fails:

import reframe as rfm
import reframe.utility.sanity as sn


class Base(rfm.RunOnlyRegressionTest):
    valid_systems = ['*']
    valid_prog_environs = ['*']
    sourcesdir = None
    executable = 'echo'

    @rfm.run_before('run')
    def set_exec_opts(self):
        self.executable_opts = [self.name]

    @rfm.run_before('sanity')
    def set_sanity_check(self):
        self.sanity_patterns = sn.assert_found(rf'{self.name}', self.stdout)


@rfm.simple_test
class T0(Base):
    pass

Stacktrace:

Traceback (most recent call last):
  File "/Users/karakasv/Repositories/reframe/reframe/core/decorators.py", line 56, in _instantiate_all
    ret.append(_instantiate(cls, args))
  File "/Users/karakasv/Repositories/reframe/reframe/core/decorators.py", line 43, in _instantiate
    return cls(_rfm_use_params=True)
  File "/Users/karakasv/Repositories/reframe/reframe/core/meta.py", line 199, in __call__
    obj = cls.__new__(cls, *args, **kwargs)
  File "/Users/karakasv/Repositories/reframe/reframe/core/pipeline.py", line 790, in __new__
    obj._rfm_init(name, prefix)
  File "/Users/karakasv/Repositories/reframe/reframe/core/pipeline.py", line 834, in _rfm_init
    if not os.path.isdir(os.path.join(self._prefix, self.sourcesdir)):
  File "/usr/local/Cellar/python@3.9/3.9.1_6/Frameworks/Python.framework/Versions/3.9/lib/python3.9/posixpath.py", line 90, in join
    genericpath._check_arg_types('join', a, *p)
  File "/usr/local/Cellar/python@3.9/3.9.1_6/Frameworks/Python.framework/Versions/3.9/lib/python3.9/genericpath.py", line 152, in _check_arg_types
    raise TypeError(f'{funcname}() argument must be str, bytes, or '
TypeError: join() argument must be str, bytes, or os.PathLike object, not 'NoneType'

The fix seems to be easy.

[vkarak]

This is still present in master. I will take it.