Fix parsing of empty strings for dates

src/main/java/com/onfido/JSON.java

@@ -612,9 +612,14 @@ public LocalDate read(JsonReader in) throws IOException {
                 case NULL:
                     in.nextNull();
                     return null;
-                default:
+                case STRING:
                     String date = in.nextString();
+                    if (date.isEmpty()) {
+                        return null;
+                    }
                     return LocalDate.parse(date, formatter);
+                default:
+                    throw new IllegalStateException("Expected STRING or NULL but was " + in.peek());
             }
         }
     }

src/test/java/com/onfido/JSONTest.java

@@ -0,0 +1,16 @@
+package com.onfido;
+
+import java.time.LocalDate;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+public class JSONTest {
+
+  @Test
+  public void parsesEmptyDateStringAsNull() {
+    com.onfido.model.Applicant applicant =
+        JSON.getGson().fromJson("{\"dob\": \"\"}", com.onfido.model.Applicant.class);
+
+    Assertions.assertNull(applicant.getDob());
+  }
+}