fix: prevent empty matrix values when no projects change

When only non-code files (like README.md) change, the change detection
was outputting [""] instead of []. This caused jobs to run with empty
matrix values, leading to failures:
- cargo test failing with "could not find Cargo.toml"
- dotnet restore failing with "no project or solution file"

Now properly outputs [] when no projects are detected, preventing
jobs from running with empty matrix values.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: markkovari

.github/workflows/tests.yml

@@ -69,9 +69,23 @@ jobs:
               fi
             done
 
-            RUST_PROJECTS=$(printf '%s\n' "${RUST_PROJECTS[@]}" | jq -R . | jq -s -c .)
-            GO_PROJECTS=$(printf '%s\n' "${GO_PROJECTS[@]}" | jq -R . | jq -s -c .)
-            CSHARP_PROJECTS=$(printf '%s\n' "${CSHARP_PROJECTS[@]}" | jq -R . | jq -s -c .)
+            if [ ${#RUST_PROJECTS[@]} -eq 0 ]; then
+              RUST_PROJECTS="[]"
+            else
+              RUST_PROJECTS=$(printf '%s\n' "${RUST_PROJECTS[@]}" | jq -R . | jq -s -c .)
+            fi
+
+            if [ ${#GO_PROJECTS[@]} -eq 0 ]; then
+              GO_PROJECTS="[]"
+            else
+              GO_PROJECTS=$(printf '%s\n' "${GO_PROJECTS[@]}" | jq -R . | jq -s -c .)
+            fi
+
+            if [ ${#CSHARP_PROJECTS[@]} -eq 0 ]; then
+              CSHARP_PROJECTS="[]"
+            else
+              CSHARP_PROJECTS=$(printf '%s\n' "${CSHARP_PROJECTS[@]}" | jq -R . | jq -s -c .)
+            fi
           fi
 
           echo "rust_projects=$RUST_PROJECTS" >> $GITHUB_OUTPUT