ICU-22758 Handle singleton decompostion to backward-combining BMP sta…

…rter

The don't exist as of Unicode 16 alpha.

icu4c/source/tools/icuexportdata/icuexportdata.cpp

@@ -1010,6 +1010,13 @@ void computeDecompositions(const char* basename,
                     nonInitialStarter = true;
                 }
             }
+            if (len == 1) {
+                // The format doesn't allow for length 1 for BMP,
+                // so if these ever occur, they need to be promoted
+                // to wider storage. As of Unicode 16 alpha, this
+                // case does not arise.
+                supplementary = true;
+            }
             if (!supplementary) {
                 if (len > LONGEST_ENCODABLE_LENGTH_16 || !len || len == 1) {
                     if (len == 18 && c == 0xFDFA) {