Reactor: fix traceback when salt:// path is nonexistant

[terminalmage]

The get_file() function is what is run when a minion requests a file
using cp.cache_file. The expected return data from cp.cache_file is a
string, not a bool. This is likely the root cause of #36121.

[cachedout]

There are two tests here that look like they were written to expect a bool.

[terminalmage]

@cachedout I'll take a look at them. If it is indeed the case that a False result is needed in those instances, we will need to modify cp.cache_file such that it returns an empty string when the result is False. But it might also be a case of writing the test to fit the behavior. I'll figure out which is the case.

[cachedout]

@terminalmage Many thanks as always.

[terminalmage]

@cachedout I looked back and it seems like cp.cache_file was modified for 2015.5.0 to return False instead of an empty string. So, I'm not convinced that this is the best change. I talked to Tom and he agreed, so I removed that commit and instead modified the reactor.