trace2: prevent segfault on config collection where no value specified

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 a null check to prevent the segfault and instead return
the "empty config value" error.

t/t0210-trace2-normal.sh

@@ -244,6 +244,14 @@ test_expect_success 'bug messages followed by BUG() are written to trace2' '
 	test_cmp expect actual
 '
 
+test_expect_success 'empty configuration values are handled' '
+  test_when_finished "rm trace2.normal actual expect" &&
+  echo >expect &&
+  GIT_TRACE2="$(pwd)/trace2.normal"  GIT_TRACE2_CONFIG_PARAMS=foo.empty \
+	  git -c foo.empty config foo.empty >actual &&
+	test_cmp expect actual
+'
+
 sane_unset GIT_TRACE2_BRIEF
 
 # Now test without environment variables and get all Trace2 settings

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)
+	if (!trace2_enabled || !value)
 		return;
 
 	redacted = redact_arg(value);