[MS] UnboundLocalError exception in MSTransferor
**Impact of the bug**
MSTransferor

**Describe the bug**
A somewhat harmless error in the service execution, but we better get it fixed.
Checking the latest tag in testbed, I see this exception has been raised once [1], so crashing the cycle with:
```
UnboundLocalError: local variable 'requestRecords' referenced before assignment
```

**How to reproduce it**
Hit an exception while retrieving workflows from ReqMgr2

**Expected behavior**
Fix the code such that this exception never happens again (1-line fix).

**Additional context and error message**
[1]
```
2021-07-31 12:04:52,395:ERROR:MSManager: MS daemon error: local variable 'requestRecords' referenced before assignment
Traceback (most recent call last):
  File "/data/srv/HG2108g/sw/slc7_amd64_gcc630/cms/reqmgr2ms/0.5.1.pre5/lib/python3.8/site-packages/WMCore/MicroService/MSManager.py", line 42, in daemon
    func(reqStatus)
  File "/data/srv/HG2108g/sw/slc7_amd64_gcc630/cms/reqmgr2ms/0.5.1.pre5/lib/python3.8/site-packages/WMCore/MicroService/MSManager.py", line 184, in transferor
    res = self.msTransferor.execute(reqStatus)
  File "/data/srv/HG2108g/sw/slc7_amd64_gcc630/cms/reqmgr2ms/0.5.1.pre5/lib/python3.8/site-packages/WMCore/MicroService/MSTransferor/MSTransferor.py", line 184, in execute
    for reqSlice in grouper(requestRecords, 100):
UnboundLocalError: local variable 'requestRecords' referenced before assignment
```
diff --git a/src/python/WMCore/MicroService/MSTransferor/MSTransferor.py b/src/python/WMCore/MicroService/MSTransferor/MSTransferor.py
index 2b9b488d3..c0c838093 100644
--- a/src/python/WMCore/MicroService/MSTransferor/MSTransferor.py
+++ b/src/python/WMCore/MicroService/MSTransferor/MSTransferor.py
@@ -160,6 +160,7 @@ class MSTransferor(MSCore):
             msg += "Service set to process up to %s requests per cycle." % self.msConfig["limitRequestsPerCycle"]
             self.logger.info(msg)
         except Exception as err:  # general error
+            requestRecords = []
             msg = "Unknown exception while fetching requests from ReqMgr2. Error: %s", str(err)
             self.logger.exception(msg)
             self.updateReportDict(summary, "error", msg)
['src/python/WMCore/MicroService/MSTransferor/MSTransferor.py']