fix: allow for uid without username

Since 1.26 (ab9a96e) we are seeing `ddsim` exceptions on certain Open Science Grid nodes/sites. Those sites appear to configure the running of singularity containers in such a way that no usernames are associated with the uid.

DDG4/python/DDSim/Helper/Meta.py

@@ -100,6 +100,9 @@ def addParametersToRunHeader(sim):
 
     # add User
     import getpass
-    runHeader["User"] = getpass.getuser()
+    try:
+        runHeader["User"] = getpass.getuser()
+    except KeyError:
+        runHeader["User"] = str(os.getuid())
 
     return runHeader