The commit under fixes added a NULL-check which prevents us from
printing text headers. Conversions to add JSON support often use:

  print_string(PRINT_FP, NULL, "some text:\n", NULL);

to print in plain text mode.

Correct output:

  Channel parameters for vpn0:
  Pre-set maximums:
  RX:		n/a
  TX:		n/a
  Other:		n/a
  Combined:	1
  Current hardware settings:
  RX:		n/a
  TX:		n/a
  Other:		n/a
  Combined:	0

With the buggy patch:

  Channel parameters for vpn0:
  RX:		n/a
  TX:		n/a
  Other:		n/a
  Combined:	1
  RX:		n/a
  TX:		n/a
  Other:		n/a
  Combined:	0

Fixes: fd328ccb3cc0 ("json_print: add NULL check before jsonw_string_field() in print_string()")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
 json_print.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/json_print.c b/json_print.c
index 4f61640392cf..e07c651f477b 100644
--- a/json_print.c
+++ b/json_print.c
@@ -143,10 +143,11 @@ void print_string(enum output_type type,
 	} else if (_IS_FP_CONTEXT(type)) {
 		if (value)
 			fprintf(stdout, fmt, value);
+		else
+			fprintf(stdout, fmt);
 	}
 }
 
-
 /*
  * value's type is bool. When using this function in FP context you can't pass
  * a value to it, you will need to use "is_json_context()" to have different
-- 
2.50.1