fix(FieldBuilders): validate schema is not null in build()

[mliem2k]

Summary

Closes #435.

All FieldBuilder implementations, from FieldBuilder14 through FieldBuilder111, build a Schema.Field with a call like this:

Schema.Field result = new Schema.Field(_name, _schema, _doc, avroFriendlyDefault, _order);

None of them ever checked that _schema had actually been set via setSchema(...) before calling into Avro's constructor. Under older Avro versions this silently produced a Field object backed by a null schema, which would only surface as a confusing failure later, somewhere downstream of build().

Avro 1.11.1 added eager default value validation inside the Schema.Field constructor itself. Because that validation needs the field's schema, a null schema now causes the constructor to throw directly, with a stack trace that points into Avro internals rather than at the actual root cause, a missing setSchema() call by the caller of FieldBuilder.

Fix

Added an explicit guard at the top of build() in every FieldBuilder implementation:

if (_schema == null) {
  throw new IllegalStateException("schema not set");
}

This fails fast with a clear, actionable message before Avro's constructor is ever invoked, and is consistent with existing validation elsewhere in the codebase, for example AbstractSchemaBuilder#build() rejecting a missing _type.

Testing

Added testBuildFailsOnNullSchema to FieldBuilderTest (helper-tests-allavro), which asserts that calling build() without ever calling setSchema() throws IllegalStateException. This test runs against every supported Avro version through the module's testAvro* tasks.

Verified locally with JDK 11 (Gradle 7.6.1, matching this repo's CI):

• ./gradlew :helper:tests:helper-tests-allavro:testAvro14 through testAvro111 (all 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 1.10, 1.11), all FieldBuilderTest cases pass, including the new regression test.
• ./gradlew :helper:helper-common:build :helper:impls:helper-impl-14:build ... :helper:impls:helper-impl-111:build (all FieldBuilder* impl modules), including checkstyle, all green.

Test plan

• testBuildFailsOnNullSchema passes across all Avro versions (1.4 through 1.11.4)
• Existing FieldBuilderTest cases continue to pass across all Avro versions
• helper-impl-* module builds (including checkstyle) pass for all affected modules

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