Fix empty custom URL validation in build.gradle.kts

[Copilot]

Custom model/tokenizer URLs passed via gradle properties were not properly validated when empty strings were provided (e.g., -PcustomPteUrl=). The Elvis operator only checks for null, not empty strings.

Changes

• Replace ?: null check with isNullOrEmpty() to catch both null and empty string cases
• Add "qwen3" to valid preset options in error message (was missing despite being defined)

Before

pteUrl = customPteUrl ?: throw GradleException("customPteUrl is required...")

After

if (customPteUrl.isNullOrEmpty()) {
    throw GradleException("customPteUrl is required...")
}
pteUrl = customPteUrl

Original prompt

If you look at llm-android.yml and UIWorkflowTest and corresponding build.gradle.kts I still feel like the model path and tokenizer path is not honored. Can you help me check?

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