Man writer: use default middle header when metadata does not...

include `header`.  This change causes pandoc to omit the middle
header parameter when `header` is not set, rather than
emitting `""`.  The parameter is optional and man will use
a default based on the section if it is not specified.

Closes #9943.

data/templates/default.man

@@ -8,7 +8,7 @@ $endif$
 $if(adjusting)$
 .ad $adjusting$
 $endif$
-.TH "$title/nowrap$" "$section/nowrap$" "$date/nowrap$" "$footer/nowrap$" "$header/nowrap$"
+.TH "$title/nowrap$" "$section/nowrap$" "$date/nowrap$" "$footer/nowrap$"$if(header)$ "$header/nowrap$"$endif$
 $for(header-includes)$
 $header-includes$
 $endfor$

test/command/9943.md

@@ -0,0 +1,20 @@
+```
+% pandoc -t man -s -Vpandoc-version=''
+---
+title: HELLO
+section: 1
+---
+^D
+.TH "HELLO" "1" "" ""
+```
+
+```
+% pandoc -t man -s -Vpandoc-version=''
+---
+title: HELLO
+section: 1
+header: Head
+---
+^D
+.TH "HELLO" "1" "" "" "Head"
+```

test/writer.man

@@ -1,4 +1,4 @@
-.TH "Pandoc Test Suite" "" "July 17, 2006" "" ""
+.TH "Pandoc Test Suite" "" "July 17, 2006" ""
 .PP
 This is a set of tests for pandoc.
 Most of them are adapted from John Gruber\[cq]s markdown test suite.