Fix cgmerge to handle call graphs with null meta field

[silas-martens]

CGMerge can now handle call graphs where the meta field is null. This is necessary to merge the patch graphs generated by cgpatch.

[jplehr]

Please add a test for this case

[silas-martens]

Please add a test for this case

From what I understand, the existing tests in cgcollector/test/ generate static call-graphs using cgcollector, which are then merged. However, since cgpatch is not yet available in the repo, and the fix relates to patch-graphs generated by it, I’m unsure where a test involving patch-graphs would fit best.

My idea is to add a separate folder inside cgcollector/test/input to hold test cases with pre-generated call-graphs that do not rely on cgcollector itself. Then, these testcases could be run as a part of run_format_two_test.sh.

[sebastiankreutzer]

As @silas-martens said, there are currently no explicit test cases for CGMerge. Adding a new test suite for this simple change seems a bit overkill.
Instead, I suggest that we abandon this fix and explicitly require CGMerge2 with CGPatch.

[jplehr]

I'm OK with requiring the new tool for the functionality that cgpatch does. It simply sounded as if this is a general bug, and so this would be a general fix.

So, my question is: What's stopping us from adding a test case that uses cgmerge and have one of the input files trigger the previous error? My understanding was that this currently crashes the tool, which I think should not happen.