Fix assignees field type in issue templates

Use an empty array instead of an empty string for the assignees field,
as required by the GitHub Issue Forms schema.

.github/ISSUE_TEMPLATE/bug_report.yml

@@ -2,7 +2,7 @@ name: Bug report
 description: Create a bug report to help us improve Invidious
 title: "[Bug] "
 labels: ["bug"]
-assignees: ""
+assignees: []
 body:
   - type: markdown
     attributes:

.github/ISSUE_TEMPLATE/enhancement.yml

@@ -2,7 +2,7 @@ name: Enhancement
 description: Suggest an enhancement for an existing feature
 title: "[Enhancement] "
 labels: ["enhancement"]
-assignees: ""
+assignees: []
 body:
   - type: markdown
     attributes:

.github/ISSUE_TEMPLATE/feature_request.yml

@@ -2,7 +2,7 @@ name: Feature request
 description: Suggest an idea for this project
 title: "[Feature request] "
 labels: ["feature-request"]
-assignees: ""
+assignees: []
 body:
   - type: markdown
     attributes: