Broken local Java version detection

[bertschneider]

The current local Java version detection in _detect_java_version may return None, which in turn breaks further processing in _local_java_repository_impl.

I encountered this problem due to a broken local Java installation, which was not able to execute java -XshowSettings:properties to begin with.

Error message:

ERROR: <builtin>: fetching _local_java_repository_rule rule //:rules_java~7.3.2~toolchains~local_jdk: Traceback (most recent call last):
    File "/private/var/tmp/_bazel_administrator/02bc2c2b51523be4b85c9cb7f6299f33/external/rules_java~7.3.2/toolchains/local_java_repository.bzl", line 217, column 72, in _local_java_repository_impl
        build_file = build_file.format(RUNTIME_VERSION = version if version.isdigit() else "0")
Error: 'NoneType' value has no field or method 'isdigit'