core: Fix assertion with empty Exec*= paths

An Exec*= line with whitespace after modifiers, like

  ExecStart=- /bin/true

is considered to have an empty command path. This is as specified, but causes
systemd to crash with

  Assertion 'skip < l' failed at ../src/core/load-fragment.c:607, function config_parse_exec(). Aborting.
  Aborted (core dumped)

Fix this by logging an error instead and ignoring the invalid line.

Add corresponding test cases. Also add a test case for a completely empty value
which resets the command list.

https://launchpad.net/bugs/1454173

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