trace2: prevent segfault on config collection with valueless true

trace2: prevent segfault on config collection with valueless true

When TRACE2 analytics is enabled, a configuration variable set to
"valueless true" 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

Add checks to prevent the segfault and instead show that the
variable without value.

Signed-off-by: Adam Murray <ad@canva.com>
Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>