Merge pull request #1716 from jedwards4b/missing_forcing_fix

this change avoids an error if the forcing description is missing from config_compsets_cesm.xml
Needed for interfacing old config_component.xml files with the new ones introduced in PR #1697

Test suite: Hand test of ERS.f09_g17.B1850Ws.cheyenne_intel.allactive-defaultio
Test baseline:
Test namelist changes:
Test status: bit for bit
Fixes

User interface changes?:

Update gh-pages html (Y/N)?: N

Code review:

scripts/lib/CIME/XML/component.py

@@ -172,9 +172,8 @@ def _get_description_v3(self, compsetname, comp_class):
                            "Too many matches on forcing field {} in file {}".\
                                format(forcing, self.filename))
                     desc = node.text
-
-            expect(len(desc) > 0,"No match found for forcing field {} in file {}".\
-                       format(forcing, self.filename))
+            if desc is None:
+                desc = compsetname.split('_')[0]
             return desc
 
         # first pass just make a hash of the option descriptions