Add a test case for mouse hover on annotation with empty content.

R=dsinclair@chromium.org

Review URL: https://codereview.chromium.org/2301793002 .
diff --git a/pdfium/annots/annotation_highlight_empty_content.evt b/pdfium/annots/annotation_highlight_empty_content.evt
new file mode 100644
index 0000000..55dadb9
--- /dev/null
+++ b/pdfium/annots/annotation_highlight_empty_content.evt
@@ -0,0 +1 @@
+mousemove,128,713
diff --git a/pdfium/annots/annotation_highlight_empty_content.pdf b/pdfium/annots/annotation_highlight_empty_content.pdf
new file mode 100644
index 0000000..61f905b
--- /dev/null
+++ b/pdfium/annots/annotation_highlight_empty_content.pdf
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diff --git a/pdfium/annots/annotation_highlight_empty_content_expected.pdf.0.png b/pdfium/annots/annotation_highlight_empty_content_expected.pdf.0.png
new file mode 100644
index 0000000..d38f9f4
--- /dev/null
+++ b/pdfium/annots/annotation_highlight_empty_content_expected.pdf.0.png
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