Inverted boolean logic in HistoryEntry.hasCorrespondingLayer()

**Environment**:

- *Jib version:* jib.core 0.20.0
- *Build tool:* N/A
- *OS:* N/A

**Description of the issue**:

Method `hasCorrespondingLayer()` of `class com.google.cloud.tools.jib.image.json.HistoryEntry` returns wrong values. It returns **true** for empty layers and vice versa.

**Expected behavior**:

Method should return false for empty layers and true for layers with content.

**Steps to reproduce**:

N/A. Sorry, just look at the one-line implementation of the method. The bug is obvious.
https://github.com/GoogleContainerTools/jib/blob/master/jib-core/src/main/java/com/google/cloud/tools/jib/image/json/HistoryEntry.java#L138-L146

**Additional Information**:

There seems to be only one use of the method within jib here:
https://github.com/GoogleContainerTools/jib/blob/0ed7dca36864b6b19ff61629fc578018041fa15f/jib-core/src/main/java/com/google/cloud/tools/jib/builder/steps/BuildImageStep.java#L96-L98

The snippet is similarly wrong:
```
if (!historyObject.hasCorrespondingLayer()) {
nonEmptyLayerCount++;
}
```
The if statement as written will trigger for empty layers, but the variable is named nonEmptyLayerCount. The logic is inverted which hides the underlying error in hasCorrespondingLayer().

hasCorrespondingLayer() needs to be fixed and this code adapted to:
```
if (historyObject.hasCorrespondingLayer()) {
nonEmptyLayerCount++;
}
```