fix: Make plain -R give an error

Author: MarcellPerger1

src/main/java/net/marcellperger/mathexpr/cli/minicli/MiniCLI.java

@@ -142,6 +142,7 @@ public void finish() {
 
         private void flushPrev() {
             Util.realAssert(prev != null, "flushPrev must have a `prev` to flush");
+            prev.setValueFromString(null);
             prev = null;
         }
         private void flushPrevWithValue(String value) {

src/main/java/net/marcellperger/mathexpr/cli/minicli/StringCLIOption.java

@@ -6,7 +6,7 @@
 import java.util.List;
 
 class StringCLIOption extends CLIOption<String> {
-    @Nullable String defaultIfNoValue;
+    @Nullable String defaultIfNoValue;  // TODO should this really be here - makes more sense in CLIOption
 
     public StringCLIOption(List<String> keys, @Nullable String defaultIfNoValue) {
         super(String.class, keys);