Fix hoomd version handling in convert_hoomd.py (#828)

* move hoomd version var under if statement

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gmso/external/convert_hoomd.py

@@ -31,6 +31,10 @@
 if has_hoomd:
     import hoomd
 
+    hoomd_version = hoomd.version.version.split(".")
+else:
+    hoomd_version = None
+
 # Note, charge will always be assumed to be in elementary_charge
 MD_UNITS = {
     "energy": u.kJ / u.mol,
@@ -44,8 +48,6 @@
     "mass": u.g / u.mol,  # aka amu
 }
 
-hoomd_version = hoomd.version.version.split(".")
-
 
 def to_gsd_snapshot(
     top,