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 an "empty config value" error.

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

Author: ad-murray

config.c

@@ -615,6 +615,8 @@ static int config_parse_pair(const char *key, const char *value,
 
 	if (!strlen(key))
 		return error(_("empty config key"));
+	if (!value || !strlen(value))
+		return error(_("empty config value"));
 	if (git_config_parse_key(key, &canonical_name, NULL))
 		return -1;