apk-size: report clear error message when "sdk" string cannot be parsed (#7206)

This PR fixes an obscure error that I cannot reproduce.

The error occurred during a github actions workflow and looked like
this:

```
[I] fireci.gradle: FAILURE: Build failed with an exception.
[I] fireci.gradle: 
[I] fireci.gradle: * Where:
[I] fireci.gradle: Script '/home/runner/work/firebase-android-sdk/firebase-android-sdk/health-metrics/apk-size/app/configure.gradle' line: 27
[I] fireci.gradle: 
[I] fireci.gradle: * What went wrong:
[I] fireci.gradle: A problem occurred evaluating script.
[I] fireci.gradle: > Index 1 out of bounds for length 1
```


https://github.com/firebase/firebase-android-sdk/actions/runs/16633221757/job/47122970800

With this fix, the error is clearer:

```
$ cd health-metrics/apk-size
$ ./gradlew -Psdks=foobar1,foobar2 tasks

FAILURE: Build failed with an exception.

* Where:
Script 'health-metrics/apk-size/app/configure.gradle' line: 30

* What went wrong:
A problem occurred evaluating script.
> Invalid SDK format: 'foobar1'. Expected format: 'groupId:artifactId:version' (sdks: 'foobar1,foobar2')
```

Author: dconeybe

ci/fireci/fireciplugins/binary_size.py

@@ -55,6 +55,13 @@ def binary_size(pull_request, log, metrics_service_url, access_token):
   affected_artifacts, all_artifacts = _parse_artifacts()
   artifacts = affected_artifacts if pull_request else all_artifacts
   sdks = ','.join(artifacts)
+  if not sdks:
+    _logger.info(
+      "No sdks found whose binary size to measure ("
+      "pull_request=%s affected_artifacts=%s all_artifacts=%s)",
+      pull_request, affected_artifacts, all_artifacts
+    )
+    return
 
   workdir = 'health-metrics/apk-size'
   process = gradle.run('assemble', '--continue', gradle.P('sdks', sdks), workdir=workdir, check=False)

health-metrics/apk-size/app/configure.gradle

@@ -24,7 +24,14 @@ repositories {
 if (project.hasProperty('sdks')) {
     project.android {
         sdks.split(',').each { sdk ->
-            def (groupId, artifactId, version) = sdk.split(':')
+            def sdkParts = sdk.split(':')
+            if (sdkParts.size() != 3) {
+                throw new GradleException(
+                    "Invalid SDK format: '${sdk}'. " +
+                    "Expected format: 'groupId:artifactId:version' (sdks: '${sdks}')"
+                )
+            }
+            def (groupId, artifactId, version) = sdkParts
             productFlavors.create(artifactId) {
                 dimension 'firebase'
             }