CI: another kind of blank

Author: cbm755

.github/workflows/main.yml

@@ -87,7 +87,7 @@ jobs:
       matrix:
         octave: [10.3.0]
         sympy: [1.9, 1.10.1, 1.11.1, 1.12.1, 1.13.0, 1.13.3, 1.14.0]
-        mpmath:  # blank means SymPy pin in effect
+        mpmath: [""]  # blank means SymPy pin in effect
         include:
           - octave: 10.2.0
             sympy: 1.14.0