more robust reset of CLASSPATH: unset variable means "." in certain situations, e.g. scalac;

--- a/bin/isabelle_java	Wed Oct 19 16:01:07 2022 +0200
+++ b/bin/isabelle_java	Thu Oct 20 14:59:39 2022 +0200
@@ -64,7 +64,7 @@
     exit 127
   else
     unset ISABELLE_HOME
-    unset CLASSPATH
+    export CLASSPATH=""
     exec "$JAVA_HOME/bin/java" "${JAVA_ARGS[@]}" \
       -classpath "$ISABELLE_CLASSPATH" "$@"
   fi

--- a/lib/Tools/java	Wed Oct 19 16:01:07 2022 +0200
+++ b/lib/Tools/java	Thu Oct 20 14:59:39 2022 +0200
@@ -7,7 +7,7 @@
 eval "declare -a JAVA_ARGS=($ISABELLE_JAVA_SYSTEM_OPTIONS)"
 
 classpath "$ISABELLE_SETUP_CLASSPATH"; unset ISABELLE_SETUP_CLASSPATH
-classpath "$CLASSPATH"; unset CLASSPATH
+classpath "$CLASSPATH"; export CLASSPATH=""
 
 isabelle_java java "${JAVA_ARGS[@]}" \
   -classpath "$(platform_path "$ISABELLE_CLASSPATH")" "$@"

--- a/lib/Tools/scala	Wed Oct 19 16:01:07 2022 +0200
+++ b/lib/Tools/scala	Thu Oct 20 14:59:39 2022 +0200
@@ -7,7 +7,7 @@
 isabelle scala_build || exit $?
 
 classpath "$ISABELLE_SETUP_CLASSPATH"; unset ISABELLE_SETUP_CLASSPATH
-classpath "$CLASSPATH"; unset CLASSPATH
+classpath "$CLASSPATH"; export CLASSPATH=""
 
 export jvm_cp_args="$(platform_path "$ISABELLE_CLASSPATH")"
 export JAVA_OPTS="$ISABELLE_JAVA_SYSTEM_OPTIONS -J-Dscala.usejavacp=true"

--- a/lib/Tools/scala_build	Wed Oct 19 16:01:07 2022 +0200
+++ b/lib/Tools/scala_build	Thu Oct 20 14:59:39 2022 +0200
@@ -57,7 +57,7 @@
   "$ISABELLE_HOME/lib/classes/Pure.shasum" \
   "$ISABELLE_HOME/src/Tools/jEdit/dist"
 
-classpath "$CLASSPATH"; unset CLASSPATH
+classpath "$CLASSPATH"; export CLASSPATH=""
 
 eval "declare -a JAVA_ARGS=($ISABELLE_TOOL_JAVA_OPTIONS)"
 

--- a/lib/Tools/scalac	Wed Oct 19 16:01:07 2022 +0200
+++ b/lib/Tools/scalac	Thu Oct 20 14:59:39 2022 +0200
@@ -7,7 +7,7 @@
 isabelle scala_build || exit $?
 
 classpath "$ISABELLE_SETUP_CLASSPATH"; unset ISABELLE_SETUP_CLASSPATH
-classpath "$CLASSPATH"; unset CLASSPATH
+classpath "$CLASSPATH"; export CLASSPATH=""
 
 isabelle_scala scalac -Dfile.encoding=UTF-8 \
   -classpath "$(platform_path "$ISABELLE_CLASSPATH")" "$@"

--- a/lib/scripts/getsettings	Wed Oct 19 16:01:07 2022 +0200
+++ b/lib/scripts/getsettings	Thu Oct 20 14:59:39 2022 +0200
@@ -41,7 +41,7 @@
   ISABELLE_ROOT="$(platform_path "$ISABELLE_HOME")"
 
   ISABELLE_CLASSPATH="$(cygpath -i -u -p "$CLASSPATH")"
-  unset CLASSPATH
+  export CLASSPATH=""
 else
   if [ -z "$USER_HOME" ]; then
     USER_HOME="$HOME"
@@ -50,7 +50,7 @@
   ISABELLE_ROOT="$ISABELLE_HOME"
 
   ISABELLE_CLASSPATH="$CLASSPATH"
-  unset CLASSPATH
+  export CLASSPATH=""
 fi
 
 #init cumulative settings