NullPointerException when processing file without GC Events

**Describe the bug**
A NullPointerException is thrown when processing a file that contains no GC events.  

The JVM does not return after this NPE occurs. (Have to kill the process manually)

**Callstack:**
```
java.lang.NullPointerException: Cannot invoke "com.microsoft.gctoolkit.time.DateTimeStamp.add(double)" because the return value of "com.microsoft.gctoolkit.parser.GCLogParser.getClock()" is null
        at com.microsoft.gctoolkit.parser@3.0.5-SNAPSHOT/com.microsoft.gctoolkit.parser.GCLogParser.receive(GCLogParser.java:97)
        at com.microsoft.gctoolkit.parser@3.0.5-SNAPSHOT/com.microsoft.gctoolkit.parser.GCLogParser.receive(GCLogParser.java:21)
        at com.microsoft.gctoolkit.vertx@3.0.5-SNAPSHOT/com.microsoft.gctoolkit.vertx.DataSourceVerticle.lambda$start$0(DataSourceVerticle.java:36)
        at io.vertx.core@4.5.7/io.vertx.core.impl.ContextInternal.dispatch(ContextInternal.java:279)
        at io.vertx.core@4.5.7/io.vertx.core.eventbus.impl.MessageConsumerImpl.dispatch(MessageConsumerImpl.java:177)
        at io.vertx.core@4.5.7/io.vertx.core.eventbus.impl.HandlerRegistration$InboundDeliveryContext.execute(HandlerRegistration.java:137)
        at io.vertx.core@4.5.7/io.vertx.core.eventbus.impl.DeliveryContextBase.next(DeliveryContextBase.java:80)
        at io.vertx.core@4.5.7/io.vertx.core.eventbus.impl.DeliveryContextBase.dispatch(DeliveryContextBase.java:43)
        at io.vertx.core@4.5.7/io.vertx.core.eventbus.impl.HandlerRegistration.dispatch(HandlerRegistration.java:98)
        at io.vertx.core@4.5.7/io.vertx.core.eventbus.impl.MessageConsumerImpl.deliver(MessageConsumerImpl.java:183)
        at io.vertx.core@4.5.7/io.vertx.core.eventbus.impl.MessageConsumerImpl.doReceive(MessageConsumerImpl.java:168)
        at io.vertx.core@4.5.7/io.vertx.core.eventbus.impl.HandlerRegistration.lambda$receive$0(HandlerRegistration.java:49)
        at io.netty.common@4.1.108.Final/io.netty.util.concurrent.AbstractEventExecutor.runTask(AbstractEventExecutor.java:173)
        at io.netty.common@4.1.108.Final/io.netty.util.concurrent.AbstractEventExecutor.safeExecute(AbstractEventExecutor.java:166)
        at io.netty.common@4.1.108.Final/io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:470)
        at io.netty.transport@4.1.108.Final/io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:569)
        at io.netty.common@4.1.108.Final/io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:997)
        at io.netty.common@4.1.108.Final/io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)
        at io.netty.common@4.1.108.Final/io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
        at java.base/java.lang.Thread.run(Thread.java:833)
```

**Sample File**
```
Java HotSpot(TM) 64-Bit Server VM (25.202-b08) for linux-amd64 JRE (1.8.0_202-b08), built on Dec 15 2018 12:40:22 by "java_re" with gcc 7.3.0
Memory: 4k page, physical 16264752k(377768k free), swap 0k(0k free)
CommandLine flags: -XX:+DisableExplicitGC -XX:GCLogFileSize=134217728 -XX:InitialHeapSize=150994944 -XX:+ManagementServer -XX:MaxHeapSize=150994944 -XX:NumberOfGCLogFiles=4 -XX:ParallelGCThreads=2 -XX:+PrintGC -XX:+PrintGCDateStamps -XX:+PrintGCDetails -XX:+PrintGCTimeStamps -XX:-TieredCompilation -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseGCLogFileRotation -XX:+UseParallelGC 
Heap
 PSYoungGen      total 43008K, used 12467K [0x00000000fd000000, 0x0000000100000000, 0x0000000100000000)
  eden space 36864K, 33% used [0x00000000fd000000,0x00000000fdc2cda0,0x00000000ff400000)
  from space 6144K, 0% used [0x00000000ffa00000,0x00000000ffa00000,0x0000000100000000)
  to   space 6144K, 0% used [0x00000000ff400000,0x00000000ff400000,0x00000000ffa00000)
 ParOldGen       total 98304K, used 0K [0x00000000f7000000, 0x00000000fd000000, 0x00000000fd000000)
  object space 98304K, 0% used [0x00000000f7000000,0x00000000f7000000,0x00000000fd000000)
 Metaspace       used 7756K, capacity 7884K, committed 7936K, reserved 1056768K
  class space    used 935K, capacity 986K, committed 1024K, reserved 1048576K
```

**To Reproduce**
Steps to reproduce the behavior:
1: Using the sample application, attempt to analyze the file above.
`mvnw.cmd exec:exec -DgcLogFile=sample.out`
2: NPE above is thrown.  Process hangs in place until terminated manually (Ctrl-C, or killing the JVM)

**Expected behavior**
File is successfully analyzed (even if no data is available) or a suitable error is returned. 

**Desktop (please complete the following information):**
 - OS: Windows 11
 - GCToolkit Version: Tip commit f9c62c6f3d9e7cdc3fccd2391eedb6b034b5d25c (#350)
 - Java: Reproduced on Java 17 and Java 11.


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