[debugify] Add targeted test for 2fa656c, NFC

https://reviews.llvm.org/D78411 introduced test changes which relied on
the ability to strip debugify metadata even if module-level metadata is
missing. This introduces a more targeted test for that ability.

llvm/test/CodeGen/Generic/MIRStripDebug/no-metadata-present.mir

@@ -0,0 +1,7 @@
+# RUN: llc -run-pass=mir-strip-debug -mir-strip-debugify-only=0 -o /dev/null %s
+
+---
+name:            test
+body:             |
+  bb.1:
+...