When TRACE2 analytics is enabled, a git config option that has no value

causes a segfault.

Steps to Reproduce
GIT_TRACE2=true GIT_TRACE2_CONFIG_PARAMS=status.*
git -c status.relativePaths version
Expected Result
git version 2.46.0
Actual Result
zsh: segmentation fault GIT_TRACE2=true

This adds checks to prevent the segfault and instead return
an empty value.

Signed-off-by: Adam Murray <[email protected]>

trace2.c

@@ -759,7 +759,7 @@ void trace2_def_param_fl(const char *file, int line, const char *param,
 	int j;
 	const char *redacted;
 
-	if (!trace2_enabled || !value)
+	if (!trace2_enabled)
 		return;
 
 	redacted = value ? redact_arg(value): NULL;