fix NPE when parsing null options used by conditions (fixes #133)

Signed-off-by: cs-cat <[email protected]>

src/main/java/pascal/taie/config/Configs.java

@@ -95,7 +95,7 @@ static boolean satisfyConditions(String conditions, AnalysisOptions options) {
                     // Check each individual value, if one match,
                     // then this condition can be satisfied.
                     for (String v : value.split("\\|")) {
-                        if (options.get(key).toString().equals(v)) {
+                        if (Objects.toString(options.get(key), "null").equals(v)) {
                             continue outer;
                         }
                     }