merge-request caused-by event displays partial json payload.

### Jenkins and plugins versions report

Environment

```text
Jenkins: 2.401.1
OS: Linux - 5.15.0-1029-azure
Java: 11.0.19 - Ubuntu (OpenJDK 64-Bit Server VM)
---
Office-365-Connector:4.18.0
ace-editor:1.1
analysis-model-api:11.3.0
ansicolor:1.0.2
ant:487.vd79d090d4ea_e
antisamy-markup-formatter:159.v25b_c67cd35fb_
apache-httpcomponents-client-4-api:4.5.14-150.v7a_b_9d17134a_5
artifactory:3.18.3
atlassian-jira-software-cloud:2.0.7
audit-trail:333.vb_e1b_b_0f1238c
authentication-tokens:1.53.v1c90fd9191a_b_
aws-credentials:191.vcb_f183ce58b_9
aws-java-sdk-ec2:1.12.481-392.v8b_291cfcda_09
aws-java-sdk-minimal:1.12.481-392.v8b_291cfcda_09
azure-ad:349.vc02b_a_0b_142a_8
azure-credentials:254.v64da_8176c83a
azure-sdk:132.v62b_48eb_6f32f
azure-vm-agents:859.v7213476e4fea_
badge:1.9.1
basic-branch-build-strategies:71.vc1421f89888e
bitbucket:223.vd12f2bca5430
blueocean:1.27.4
blueocean-autofavorite:1.2.5
blueocean-bitbucket-pipeline:1.27.4
blueocean-commons:1.27.4
blueocean-config:1.27.4
blueocean-core-js:1.27.4
blueocean-dashboard:1.27.4
blueocean-display-url:2.4.2
blueocean-events:1.27.4
blueocean-git-pipeline:1.27.4
blueocean-github-pipeline:1.27.4
blueocean-i18n:1.27.4
blueocean-jwt:1.27.4
blueocean-personalization:1.27.4
blueocean-pipeline-api-impl:1.27.4
blueocean-pipeline-editor:1.27.4
blueocean-pipeline-scm-api:1.27.4
blueocean-rest:1.27.4
blueocean-rest-impl:1.27.4
blueocean-web:1.27.4
bootstrap5-api:5.3.0-1
bouncycastle-api:2.28
branch-api:2.1109.vdf225489a_16d
build-monitor-plugin:1.14-717.v3efcdffe8d58
build-time-blame:70.v5031def3c2e3-2.1.0
build-timeout:1.31
build-timestamp:1.0.3
buildtriggerbadge:251.vdf6ef853f3f5
caffeine-api:3.1.6-115.vb_8b_b_328e59d8
change-assembly-version-plugin:1.10
checks-api:2.0.0
claim:516.v36293563731d
cloud-stats:267.v577e3742c282
cloudbees-bitbucket-branch-source:809.vc1d904b_30426
cloudbees-folder:6.815.v0dd5a_cb_40e0e
cobertura:1.17
code-coverage-api:4.7.0
command-launcher:100.v2f6722292ee8
commons-httpclient3-api:3.1-3
commons-lang3-api:3.12.0-36.vd97de6465d5b_
commons-text-api:1.10.0-36.vc008c8fcda_7b_
config-file-provider:938.ve2b_8a_591c596
configuration-as-code:1647.ve39ca_b_829b_42
copyartifact:705.v5295cffec284
credentials:1254.vb_96f366e7b_a_d
credentials-binding:604.vb_64480b_c56ca_
custom-tools-plugin:0.8
dark-theme:336.v02165cd8c2ee
dashboard-view:2.487.vcf0ff9008a_c0
data-tables-api:1.13.4-2
datadog:5.4.1
display-url-api:2.3.7
docker-commons:419.v8e3cd84ef49c
docker-workflow:563.vd5d2e5c4007f
dropdown-viewstabbar-plugin:1.7
dtkit-api:3.0.2
durable-task:507.v050055d0cb_dd
echarts-api:5.4.0-5
email-ext:2.99
emailext-template:1.5
embeddable-build-status:385.vc95f94e91fb_3
extended-choice-parameter:373.v1a_ecea_fdf2a_a_
extended-read-permission:53.v6499940139e5
external-monitor-job:203.v683c09d993b_9
favorite:2.4.2
font-awesome-api:6.4.0-1
forensics-api:2.3.0
git:5.1.0
git-changelog:3.30
git-client:4.4.0
git-forensics:2.0.0
github:1.37.1
github-api:1.314-431.v78d72a_3fe4c3
github-branch-source:1728.v859147241f49
gitlab-api:5.3.0-88.vf8787b_92d679
gitlab-branch-source:663.v2602c3e6376d
gitlab-plugin:1.7.14
gradle:2.8
groovy:453.vcdb_a_c5c99890
h2-api:11.1.4.199-12.v9f4244395f7a_
handlebars:3.0.8
handy-uri-templates-2-api:2.1.8-22.v77d5b_75e6953
hashicorp-vault-pipeline:1.4
hashicorp-vault-plugin:360.v0a_1c04cf807d
htmlpublisher:1.31
implied-labels:228.v63b_3f8443d4e
instance-identity:173.va_37c494ec4e5
ionicons-api:56.v1b_1c8c49374e
ivy:2.5
jackson2-api:2.15.2-350.v0c2f3f8fc595
jakarta-activation-api:2.0.1-3
jakarta-mail-api:2.0.1-3
javadoc:233.vdc1a_ec702cff
javax-activation-api:1.2.0-6
javax-mail-api:1.6.2-9
jaxb:2.3.8-1
jdk-tool:66.vd8fa_64ee91b_d
jenkins-design-language:1.27.4
jersey2-api:2.39.1-2
jfrog:1.4.0
jira:3.10
jjwt-api:0.11.5-77.v646c772fddb_0
jnr-posix-api:3.1.17-1
jobConfigHistory:1212.vd4470d08ff12
jquery:1.12.4-1
jquery3-api:3.7.0-1
jsch:0.2.8-65.v052c39de79b_2
junit:1207.va_09d5100410f
kubernetes:3937.vd7b_82db_e347b_
kubernetes-client-api:6.4.1-215.v2ed17097a_8e9
kubernetes-credentials:0.10.0
label-linked-jobs:6.0.1
ldap:682.v7b_544c9d1512
lockable-resources:1166.v7ef984f72fe2
mailer:457.v3f72cb_e015e5
markdown-formatter:95.v17a_965e696ee
matrix-auth:3.1.8
matrix-project:789.v57a_725b_63c79
maven-plugin:3.22
mercurial:1260.vdfb_723cdcc81
metrics:4.2.18-439.v86a_20b_a_8318b_
mina-sshd-api-common:2.10.0-69.v28e3e36d18eb_
mina-sshd-api-core:2.10.0-69.v28e3e36d18eb_
modernstatus:1.3
momentjs:1.1.1
monitoring:1.94.1
msbuild:1.30
nodejs:1.6.0
nunit:394.v65c09a_8408b_a_
okhttp-api:4.11.0-145.vcb_8de402ef81
pam-auth:1.10
parameterized-scheduler:1.2
parameterized-trigger:2.45
pipeline-aggregator-view:1.15
pipeline-build-step:496.v2449a_9a_221f2
pipeline-graph-analysis:202.va_d268e64deb_3
pipeline-groovy-lib:656.va_a_ceeb_6ffb_f7
pipeline-input-step:468.va_5db_051498a_4
pipeline-maven:1298.v43b_82f220a_e9
pipeline-milestone-step:111.v449306f708b_7
pipeline-model-api:2.2141.v5402e818a_779
pipeline-model-definition:2.2141.v5402e818a_779
pipeline-model-extensions:2.2141.v5402e818a_779
pipeline-npm:95.v5213efa_9585f
pipeline-rest-api:2.32
pipeline-stage-step:305.ve96d0205c1c6
pipeline-stage-tags-metadata:2.2141.v5402e818a_779
pipeline-stage-view:2.32
pipeline-utility-steps:2.15.4
plain-credentials:143.v1b_df8b_d3b_e48
plugin-util-api:3.3.0
popper2-api:2.11.6-2
postgresql-api:42.3.3
prism-api:1.29.0-7
project-stats-plugin:23.v47fee1f77b_84
pubsub-light:1.17
resource-disposer:0.22
role-strategy:633.v836e5b_3e80a_5
scm-api:676.v886669a_199a_a_
script-security:1251.vfe552ed55f8d
simple-theme-plugin:160.vb_76454b_67900
slack:664.vc9a_90f8b_c24a_
snakeyaml-api:1.33-95.va_b_a_e3e47b_fa_4
sse-gateway:1.26
ssh-credentials:305.v8f4381501156
sshd:3.303.vefc7119b_ec23
structs:324.va_f5d6774f3a_d
swarm:3.40
terraform:1.0.10
theme-manager:193.vcef22f6c5f2b_
timestamper:1.25
token-macro:359.vb_cde11682e0c
trilead-api:2.84.v72119de229b_7
variant:59.vf075fe829ccb
view-job-filters:364.v48a_33389553d
warnings-ng:10.2.0
windows-slaves:1.8.1
workflow-aggregator:596.v8c21c963d92d
workflow-api:1215.v2b_ee3e1b_dd39
workflow-basic-steps:1017.vb_45b_302f0cea_
workflow-cps:3691.v28b_14c465a_b_b_
workflow-durable-task-step:1247.v7f9dfea_b_4fd0
workflow-job:1308.v58d48a_763b_31
workflow-multibranch:756.v891d88f2cd46
workflow-scm-step:415.v434365564324
workflow-step-api:639.v6eca_cd8c04a_a_
workflow-support:839.v35e2736cfd5c
ws-cleanup:0.45
xunit:3.1.2
```

### What Operating System are you using (both controller, and any agents involved in the problem)?

Ubuntu Linux (20.04), either in docker or in Azure.

### Reproduction steps

1. Create repo in gitlab
2. Create analog job in jenkins
3. Create branch and make changes.
4. Open a merge-request
5. Make a _code change_ to said branch / merge request
6. See the build even reason in Jenkins as something like

### Expected Results

The title of the merge-request build would be something like:
`Triggered by GitLab Merge Request #1: project-xxxxx => main`

### Actual Results

The title of the merge-request build is:
```
[Merge request !1 state:opened action:update changes:{ "authorId" : null, "createdAt" : null, "updatedAt" : { "previous" : "2023-06-16T12:31:07Z", "current" : "2023-06-16T14:11:28Z" }, "updatedById" : null, "title" : null, "description" : null, "state" : null, "milestoneId" : null, "labels" : null, "assignees" : null, "totalTimeSpent" : null, "mergeStatus" : null, "reviewers" : null } at 10:11:29 AM on Jun 16, 2023](https://jenkins.company.site/terraform-module/indexing/events)
```

### Anything else?

This is Gitlab 16.0.1 (debian package)