[lit] Don't assume you'll find debugserver

On Linux, there is no "debugserver" process, and the RUN-line substitution will
fail if you try to substitute '%debugserver' with None.

Fixes PR30492.

llvm-svn: 283520

lldb/lit/lit.cfg

@@ -140,7 +140,9 @@ config.substitutions.append(('%cc', config.cc))
 config.substitutions.append(('%cxx', config.cxx))
 
 config.substitutions.append(('%lldb', lldb))
-config.substitutions.append(('%debugserver', debugserver))
+
+if debugserver is not None:
+    config.substitutions.append(('%debugserver', debugserver))
 
 for pattern in [r"\bFileCheck\b",
                 r"\| \bnot\b"]: