fix: Guard signingConfigs behind key.properties existence check

[sunbird89629]

Summary

• Wrap signingConfigs and its usage in keystorePropertiesFile.exists() check
• Prevents null cannot be cast to non-null type kotlin.String error when key.properties is missing

Problem

The example app's build.gradle.kts unconditionally accesses keystoreProperties["keyAlias"] as String in the signingConfigs block. When key.properties doesn't exist (fresh clone, local dev), this crashes with:

null cannot be cast to non-null type kotlin.String

Fix

Guard both the signingConfigs creation and its usage in buildTypes.release behind keystorePropertiesFile.exists().

Test plan

• Verify flutter build apk works without key.properties
• Verify flutter build apk --release works with valid key.properties