libelektra: highlevel: do not fail without any spec

@haraldg reported a usability problem of the highlevel API.

While it is important that we fail with an inconsistent/wrong spec during kdbOpen it is not a good design choice that elektraGet calls fail because of a missing type in the spec. Let us assume following example program:

#include <stdio.h>
#include <elektra.h>

int main () {
        Elektra *elektra = elektraOpen("/sw/test/elektraHighLevel/", NULL, NULL, NULL);

        const char *output = elektraGetString(elektra, "output");

        printf("%s", output);

        return 0;
}

Expected Behavior

It is fine to fail if there is no config nor spec:

kdb rm /sw/test/elektraHighLevel/output
./a.out
# RET: 1

Unwanted Behavior

It is weird to fail if there is no spec and there is a valid value:

kdb set /sw/test/elektraHighLevel/output "Hello World"
./a.out
# RET: 1

Guidelines

In general, we should try to fail within elektraOpen if possible (specs without defaults or value, without types). At run-time we should only fail if there is an unrecoverable error/inconsistency. (e.g. other type specified than asked for, value not according the type and so on…). Not having a spec is neither an error nor an inconsistency.