hugolib: Fix regression in summary generation behavior since v0.123

Fix regression in content summarization so that we can use empty
summary by using the manual summary divider. Since v0.123, there
has been the regression that causes Hugo to use automatic summary
generation when the manual summary results in an empty string,
even if there is a `<!--more-->` summary divider.

hugolib/page__content.go

@@ -770,7 +770,7 @@ func (c *cachedContent) contentPlain(ctx context.Context, cp *pageContentOutput)
 			result.readingTime = (result.wordCount + 212) / 213
 		}
 
-		if rendered.summary != "" {
+		if c.pi.hasSummaryDivider || rendered.summary != "" {
 			result.summary = rendered.summary
 			result.summaryTruncated = rendered.summaryTruncated
 		} else if cp.po.p.m.pageConfig.Summary != "" {

hugolib/page_test.go

@@ -63,6 +63,15 @@ Summary Next Line
 
 <!--more-->
 Some more text
+`
+
+	simplePageWithBlankSummary = `---
+title: SimpleWithBlankSummary
+---
+
+<!--more-->
+
+Some text.
 `
 
 	simplePageWithSummaryParameter = `---
@@ -351,6 +360,9 @@ func normalizeExpected(ext, str string) string {
 
 		return expected
 	case "rst":
+		if str == "" {
+			return "<div class=\"document\"></div>"
+		}
 		return fmt.Sprintf("<div class=\"document\">\n\n\n%s</div>", str)
 	}
 }
@@ -630,6 +642,19 @@ func TestPageWithDelimiter(t *testing.T) {
 	testAllMarkdownEnginesForPages(t, assertFunc, nil, simplePageWithSummaryDelimiter)
 }
 
+func TestPageWithBlankSummary(t *testing.T) {
+	t.Parallel()
+	assertFunc := func(t *testing.T, ext string, pages page.Pages) {
+		p := pages[0]
+		checkPageTitle(t, p, "SimpleWithBlankSummary")
+		checkPageContent(t, p, normalizeExpected(ext, "<p>Some text.</p>\n"), ext)
+		checkPageSummary(t, p, normalizeExpected(ext, ""), ext)
+		checkPageType(t, p, "page")
+	}
+
+	testAllMarkdownEnginesForPages(t, assertFunc, nil, simplePageWithBlankSummary)
+}
+
 func TestPageWithSummaryParameter(t *testing.T) {
 	t.Parallel()
 	assertFunc := func(t *testing.T, ext string, pages page.Pages) {