fix handling extra header correctly

build/utils/build_information.py

@@ -96,7 +96,8 @@ class BuildInfo:
                 or include_pytex_info_text
                 or include_timestamp
                 or include_pytex_version
-                or include_git_version):
+                or include_git_version
+                or extra_header):
             self._header = None
             return
         else:
@@ -113,7 +114,8 @@ class BuildInfo:
             self._header += PYTEX_VERSION
         if include_git_version:
             self._header += SOURCE_CODE_VERSION
-        self._header += ['']
+        if len(self._header) > 0:
+            self._header += ['']
         if extra_header:
             self._header += extra_header + ['']