Fix #799: empty-string custom tick labels crash TextLayout

[timmolter]

Problem

When a custom Y-axis (or X-axis) tick label formatter returns "" to suppress a label, AxisTickLabels.paint() passed the empty string directly to the TextLayout constructor, which throws IllegalArgumentException: Zero length string passed to TextLayout constructor.

Reproducer:

chart.setCustomYAxisTickLabelsFormatter(yValue -> {
    Double label = myMap.get(yValue);
    return label == null ? "" : label.toString(); // "" for unmapped values
});
BitmapEncoder.saveBitmap(chart, "output.png", BitmapEncoder.BitmapFormat.PNG); // crashes

Root Cause

AxisTickLabels.paint() checked tickLabel != null before passing labels to TextLayout, but did not check for empty strings. TextLayout accepts null but throws on "".

ColocatedSlaveLabels.paint() had the same gap (its sibling method maxSlaveWidth() already guarded for !isEmpty() correctly).

Fix

Treat "" the same as null — silently skip rendering. No label text is drawn; tick marks remain visible (consistent with the existing null/log-axis behaviour and the documented " " hide-label workaround from #792).

Files changed:

• AxisTickLabels.java — add !tickLabel.isEmpty() to the Y-axis guard and both X-axis guards
• ColocatedSlaveLabels.java — add !label.isEmpty() to paint() to match the existing guard in maxSlaveWidth()
• XYChartTest.java — replace @Disabled stub with three @Test regressions: all-duplicate labels, Y-axis empty labels, X-axis empty labels
• TestForIssue799.java — headless-safe demo class

Closes #799