Don't allow empty context keys/values (#688)

api/src/main/java/me/lucko/luckperms/api/context/AbstractContextSet.java

@@ -122,11 +122,33 @@ abstract class AbstractContextSet implements ContextSet {
     }
 
     static String sanitizeKey(String key) {
-        return Objects.requireNonNull(key, "key is null").toLowerCase().intern();
+        Objects.requireNonNull(key, "key is null");
+        if (stringIsEmpty(key)) {
+            throw new IllegalArgumentException("key is (effectively) empty");
+        }
+
+        return key.toLowerCase().intern();
     }
 
     static String sanitizeValue(String value) {
-        return Objects.requireNonNull(value, "value is null").intern();
+        Objects.requireNonNull(value, "value is null");
+        if (stringIsEmpty(value)) {
+            throw new IllegalArgumentException("value is (effectively) empty");
+        }
+
+        return value.intern();
+    }
+
+    private static boolean stringIsEmpty(String s) {
+        if (s.isEmpty()) {
+            return true;
+        }
+        for (char c : s.toCharArray()) {
+            if (c != ' ') {
+                return false;
+            }
+        }
+        return true;
     }
 
 }

api/src/main/java/me/lucko/luckperms/api/context/ContextSet.java

@@ -51,7 +51,9 @@ import javax.annotation.Nonnull;
  * {@link String#toLowerCase() lowercase} by all implementations.
  * Values however are case-sensitive.</p>
  *
- * <p>Context keys and values may not be null.</p>
+ * <p>Context keys and values may not be null or empty. A key/value will be
+ * deemed empty if it's length is zero, or if it consists of only space
+ * characters.</p>
  *
  * <p>Two default ContextSet implementations are provided.
  * {@link MutableContextSet} allows the addition and removal of context keys