Handle corner-case of empty testcase.
Currently, according the DTD definition (https://gitlab.com/sosy-lab/software/test-format/blob/master/testcase.dtd), test cases are allowed to contain no input lines at all.
This makes sense, e.g. the following benchmark https://github.com/sosy-lab/sv-benchmarks/blob/master/c/ldv-regression/alias_of_return.c_1.i does not rely on such input.
Currently, this is handled as an invalid test case which is contradicting to the dtd. I guess, one or the other should be fixed.