[C++] Extend IndexError's logic for empty arrays

include/cantera/base/ctexceptions.h

@@ -180,7 +180,8 @@ class IndexError : public CanteraError
      * @param arrayName name of the corresponding array
      * @param m   This is the value of the out-of-bounds index.
      * @param mmax This is the maximum allowed value of the index. The
-     *             minimum allowed value is assumed to be 0.
+     *             minimum allowed value is assumed to be 0. The special
+     *             value npos indicates that the array is empty.
      */
     IndexError(const string& func, const string& arrayName, size_t m, size_t mmax) :
         CanteraError(func), arrayName_(arrayName), m_(m), mmax_(mmax) {}

src/base/ctexceptions.cpp

@@ -72,6 +72,10 @@ string ArraySizeError::getMessage() const
 
 string IndexError::getMessage() const
 {
+    if (mmax_ == npos) {
+        return fmt::format("IndexError: index {} given, but array{} is empty.",
+                           m_, arrayName_.empty() ? arrayName_ : " "+arrayName_);
+    }
     if (arrayName_ == "") {
         return fmt::format("IndexError: {} outside valid range of 0 to {}.", m_, mmax_);
     }